You can ask for a substitution of anything for anything else that is on the menu, plus if it's on the menu at one of the other restaurants as, at least on the Wonder and Magic, they do share a kitchen. Also, you can order off the kids menu (no one is going to judge you) and in my experience the menu's usually had an adult 'plain fare' option near the bottom where you could get plain steak or chicken.
Disney gets a lot of picky eaters. They have ALL the experience in pleasing them!
That said, I used to be a picky eater (my husband would say still am, but much improved). I found the
Disney Cruise to be a GREAT way to expand my palate because I could order literally ANYTHING off the menu, even if I wasn't sure about it, and try it out without penalty. No one would judge me, I didn't have to eat it if I didn't like it, I wouldn't go hungry because I could easily get something else, and I wasn't spending extra money to try a bite of something I might not like! Try ordering the wasabi mashed potatoes AND the sour cream mashed potatoes! Who knows, maybe you'll like it!
Seriously, Disney Cruises are where I had my first french onion soup (awesome), first goat cheese (awesome), where I learned to like salads (awesome) and where I first tried escargot (no, no, never again no). I never would have tried these things in a regular restaurant, and would have avoided them at someone elses house so as not to insult my host by making an uncontrollable face if I didn't like them (you don't get invited back after spitting your food back onto your spoon, just sayin' and I used to do that all the time when I was a kid).