I have a hectic schedule and 2 of my 3 jobs require a lot of driving, so I sometimes have little choice about eating fast food. At Burger King I can order a Whopper junior and eat half of the bun (or less) and that works OK. At McDonalds, the same thing with a small burger (I add tomatoes to it; if you order through their app it is very easy to customize anything). Arby's roast beef is pretty good and again you can eat half the bun or none of it. I always ask for no added salt, as highly salted food really disagrees with me now and fast food places load their burgers with salt. At Wendy's, I get chili. Fries make me feel terrible and I never order them; may eat 2-3 if I'm with someone eating them. I can usually find a Panera which has good salads, or I can opt for soup there and that works too. With breakfast sandwiches, again I eat at most half of the bread and sometimes just pick out the eggs and meat.
On the rare occasion that I go to a real restaurant, I will order something that is basically protein and veggies and take half of it home a lot of the time. (I had shrimp fajitas yesterday; ate all the shrimp, some of the veggies and beans, no rice or tortillas.) If there is a heavy sauce I ask for it on the side and eat very little of it. Sometimes I order an appetizer if they have something appropriate, or a salad with protein. When I am with my son, we order one appetizer and one entree that are made of things I can tolerate and I eat around 20% of each. I don't ask for "no butter" because I can tolerate that as long as the food isn't drowning in it. I find I only have to ask for no added salt at fast food or low-end restaurants; a good restaurant will use an amount of salt that's not objectionable to me. Your mileage may vary.