DS doesn't eat a lot of meat, usually, either but I have found he can polish off almost an entire lamb shank on his own

I think sometimes it is pot luck what you try them with and what they suddenly decide they like. We rarely eat lamb shank but DP makes those boil in the bag in sauce sort now and again.
Another shocker - fried sprats, yeah he eats the head, tail, eye ball, the lot, and loves every bite. In fact it was his love of sprats (and some home made fish fingers) that eventually got him to try a 'real' fish, which he will do now. Again it's pot luck what they chose to love.
Also, if you make a stew or curry it's easy to throw a can of something in, beans into stew are great (DF can pick them out if he wants) and lentils into curry.
And there are some protein rich cakes on the allergies recipe thread if you haven't seen it. Made with white beans and lots of eggs (no flour, no sugar, just some honey I think), they don't taste like beans or eggs, they taste yummy. I know long term you don't want to teach her to eat lots of cake, but if you are concerned about protein intake then perhaps cake can help?