DS2, who has ASD is a very fussy eater, but we have worked hard at increasing the range of what he eats. He's very much into brown crunchy food and has gone through phases of refusing all but carbs.
We got protein back into his diet by going down the coated with breadcrumbs route. We had days of him eating the coating and rejecting the rest, but he was eventually brave enough to try things without peeling them and realised that he liked the meat or fish inside. He will now eat meat without the coating.
Fruit, he has grown to love baked into cakes and puds. Apple pie, rhubarb crumble and blueberry muffins all go down very well. He has progressed to liking apples raw and will happily eat one 2 or 3 times a week.
Veg, we've been less than successful with. We can hide tiny amounts of carrot, pumpkin, courgette and even beetroot in cakes, but not enough to make a difference. He will eat pizza, of course, so long as there's no big lumps of veg on it. Vegetable fingers/burgers etc simply get dismantled. Even if I am not trying to hide veg, I put a tiny piece on his plate and he has at least progressed beyond refusing the whole plate of food until its removed and will actually touch it to move it out of the way, which I know is hard for him, since he will gag if we put a piece of raw broccoli or carrot in his hand.
DS1 also has ASD and is quite fussy, too, but he does at least get a balanced diet and, given that he's prone to refusing to eat entirely, we don't make a big deal about it.
I think the key with all fussy eaters is not to make it into an emotive issue or a source of stress at mealtimes. If you do, it becomes an issue of control and a battle of wills that you can't possibly win.