I wrote this in another thread but it applies here too.
I have sucessfully converted one quaker and 3 parakeets by doing the following:
1. I make a chopped veggie salad with a combination of several things. Want a list? Keep reading. If not, skip to #2.
Broccoli, cauliflower, carrots, spinach, swiss, purple, and rainbow chard, dandelion leaves (from store, not picked wild), kale, bet greens, mustard greens, kohlrabi, kiwi, apple, beets, sweet potato, jalopeno and bell peppers, zucchini and summer squash
Then I add some seeds and seedy things, and grains: chia, sesame, flax, quinoa, poppy, barley, wheat berry, hemp, celery seeds
Throw in some legumes. I love the organic 16 bean mixes, soaked and prepared accordingly. Canned will work.
Lastly, I cook some chickpea pasta just for fun. My borbs love noodles and this gives them the look of noodles minus the noodle carbs.
2. Puree small amount of this mix with a stick blender and a tiny amount of water. If you want to start with a single veggie for this, I suggest sweet potato. Add more as your lifestyle allows. I make tons at a time and freeze it.
3. Smush the disgusting looking smoothie with the seeds they're used to and sprinkle some on top. If the consistency is right it will look like mud cake. I press it good into a dish and smooth out the top. That way I can see if borb did some digging for more of the stuff they want. Lil buddy has no choice but to get a mouth full of the veggie mud cake. The sprinkled seeds are key, they might not go near it of they don't recognize it by sight as familiar food. I graduate the smoothness of the mud cake out, getting coarser as time goes on, until I don't blend it at all, as well as the amount of mixed in seeds until there are no more commercially bought seeds. I always have seedy things in the veggie salad anyway, so they're not deprived of them. I feed the salad in the morning, and a commercially produced pellet mid-late afternoon. One little munchkin took only a week to convert, but one took a whole month. I go slow and follow the pace of the bird.
Bonus: I convert to one thing at a time. Once a good veggie diet is established, I do the same with pellets. I grind some pellets into powder and sprinkle on the veggies until used to the taste. I find that offering these meals while the family is eating in the bird's sight helps with the social aspect of sharing meals and makes these transitions faster. They *want* to eat while everyone else eats. The other thing I do, even if the bird is not bonded to me, is that I pretend to nibble from their food bowl as I'm putting it down. Gets my quaker to try new things too, especially if I pretend it's mine and she can't have any.