The other thing to try (that I forgot to mention earlier, but that I buy regularly) as a kind of "substitute" for the fresh Veggies and Greens are the "Worldly-Cuisines" that Higgins makes, and that you can order online by the pouch...There are 8 different varieties/flavors of them, and I first saw them at Petco (they only sell one of the varieties in their stores, it's called "Sunset" something or other, I can't remember, but my guys all love it), so I bought one pouch for like $2-$3 of the "Sunset" variety, and they loved it, all of them at their entire bowls of it. So I went online to see if I could order them cheaper, and then saw that they have 8 different flavors...So I ordered one pouch of each flavor and we tried each of them, and they love them all! They come in little pouches that you simply tear-open a crack and then put in the microwave and heat-up, and then I empty them into different little bowls for each of them, let them cool-down a little, and they each get a little bowl of them, and not only do all 4 of my larger guys love them all, but so do all 8 of my Budgies! There's never any waste, and I'll be honest, the "Sunset" one smelled so good that I tried it, and they're all pretty good! (the "Sunset" one smells like cinnamon oatmeal or something)...They are all made-up of different veggies, greens, healthy grains (they all have Quinoa as their base, which is awesome), legumes, healthy seeds like Chia, Hemp, and Flax, etc., herbs, and spices. I was worried about their sugar-content at first because they smelled so good, but when I tasted them I could tell that they definitely had no added-sugar or salt, they are all naturally-flavored by the herbs, the Quinoa and other grains, and the spices like cinnamon, nutmeg, red bell pepper, etc. They are a nice change of pace for them from just their regular fresh veggies and greens, and much healthier for them all the way around than any fresh fruit you could give them as a treat, because they are LOADED with lots of protein, vitamins, minerals, amino-acids/digestive enzymes, and they all have added Probiotics (acidophilus) in them as well...And they are all low fat, low sugar, low sodium...So they're not only good and they look at them like they're treats, but they're good for them too.
We started buying the Worldly Cuisines in large cases at the Avian Rescue I work for (most companies will give Rescues and other 501c3 non-profit animal services a wholesale account for all of their products, from food to cages to toys, etc., and we already had one with Higgins because we feed their low-fat/high-protein "California Blend" seed-mix to the birds who first arrive as "seed-addicts" on junky, cheap seed-mixes), and they are now feeding them to all the birds as one of their weekly servings of fruit, and the birds we have that won't touch any veggies, greens, or even fruit (we get A LOT of birds that won't even touch a single piece of fruit, like a strawberry or blueberry, nothing, let alone eating veggies or greens), they get a portion of them once daily to provide them with the extra protein and other nutrition. So they are definitely worth checking-out, since the entire point of feeding pet/captive birds and parrots fresh veggies and greens is so they get the added/extra nutrition from them that their staple-diets may not supply, and they are very good for them and contain nothing but healthy, good stuff for them..And they seem to work well on the stubborn birds...If nothing else, swing by your local Petco sometime and just buy one pouch of the "Sunset" variety that they sell in their stores and see if your guy likes it; if he/she hates it you're only out a couple of bucks.. (I'm sorry, I am blanking on the flavor, I think it's called "Sonoran Sunset", but I'm not sure, it's the only one I've ever seen sold in any of their stores around me so you can't miss it)
By the way, I'm sure you already know this, but I'm going to write it anyway because the other day I read a post where a newer member wrote about how their bird would only eat "canned veggies", and there isn't much worse than canned veggies, greens, or fruit that you could feed your bird. You always want to feed them Fresh or Freeze-Dried Veggies and Greens ONLY, because they are the only ones that retain their vitamins/minerals, or like Wrench said, just very lightly/quickly heat them for a minute and that's it, otherwise if you actually cook them through you lose all the vitamins/minerals and there's no point to even feeding them to them at that point; same goes for "canned" veggies, greens, and fruit except they are actually dangerous to feed them believe it or not...they are horrible because they contain loads and loads of salt and sugar, same with canned fruit, it's nothing but sugar because they can them all in some kind of syrup, which is nothing but sugar...And the canned veggies and greens (as well as canned legumes, like beans) are also purposely canned in a TON of preservatives so that they have an unlimited shelf-life, like Nitrates and Nitrates, along with the amount of salt you should eat in a week in one can....And I have never fed them to my birds just because I buy a ton of fresh produce every week, but I think frozen veggies and greens are okay to feed the, I wouldn't see why not, they are usually just fresh veggies/greens that are flash-frozen and put in a bag, that's it, they aren't cooked first I don't think (are they?) and nothing is added to them as far as salt, sugar, or preservatives...So you might try some frozen veggies and see if your bird will eat them as well, like some kind of frozen "mixed-veggies" variety...
Please correct me about the frozen veggies/greens if they are actually not good to feed to birds due to them having added stuff like salt/sugar/preservatives, or if they are actually cooked first before they are frozen, because that would remove all of the vitamins/minerals from them and there would be no point to feeding them to birds in the first place then...I'm just guessing because I haven't ever bought frozen veggies/greens myself, but I was always under the impression that they were just fresh veggies, greens, and fruit that were picked and then quickly flash-frozen, and that they weren't at all cooked...But I honestly don't know now that I think about it...
****One last thing you probably want to try to see if your bird likes are some of the many, many types of commercially-sold dehydrated/freeze-dried veggies and greens that are now sold in every grocery store in the country along with every Walmart, Target, etc. I'm talking about the veggie and fruit "chips" and "sticks" that they sell, such as "Green Bean Sticks", "Apple Chips", etc. There are hundreds of different brands and varieties, everything from bags of green bean sticks and sweet potato sticks to containers of dehydrated Kale...You just have to make sure that you always buy only the "Unsalted" varieties, and that you stay away from most of the different flavored ones and buy only the "Plain" varieties, because a lot of them are salted, and many of them have flavorings on them that are not good for birds, like Ranch for example. But they all come in an unsalted and plain variety, and you really could try a different type once a week for a year and not buy the same one twice, that's how many there are...Lots of dehydrated pepper varieties that birds usually love because they can't taste the heat of any type of pepper and they love the seeds...I think they are absolutely disgusting and are just like eating a shingle or a piece of cardboard, but I bought a container of that dehydrated Kale that they sell everywhere, the plain or "original" variety of it that is nothing but just pieces of dehydrated fresh Kale, just to see if they would eat it (Kale is loaded with vitamins and minerals and is very healthy to eat), and my birds all love it, so they have each been getting a piece of that a day along with their fresh veggies and greens, and they go for the "Kale chip" first every time and devour it immediately...So that's another one to try out, I first noticed this kind of stuff in Walmart when I was buying some fresh veggies and greens, they have a huge stand next to the fresh veggies and fruit that is nothing but these kind of snacks, and they sell the containers of plain, dehydrated Kale chips cheaply. I'm glad I decided to buy them and try them out, because they all really love them, with Bowie, my Green Cheek, actually attacking me as soon as I pick the container up...i have no idea why, but they love it, and it's healthy for them so whatever...
(They had a lady passing out little cups of the dehydrated Kale chips from a little stand by the fresh produce, that's how I first learned about it...I walked up to check it out because I'd never seen it before, and I couldn't tell what the hell it was just by looking at it...When the sample lady asked me if I wanted to try some "Kale chips" and handed me a little cup full of the stuff, I looked at it and then said "Am I supposed to eat this or smoke it?"...My humor totally went right over her head, she didn't get it at all and looked at me like I was insane...