Ironically, steak houses usually have the right kind of food. I order:
(1) a baked potato (always specifying "plain" or "only with chives" to avoid bacon, cheese, and sour cream as a topping).
(2) one or two side orders of vegetables if I can get them plain (no cheese sauce, for example).
Most other restaurants have menus with separate sections called "side orders," "bar menu," "salads," or "appetizers." I usually order from those sections. Examples are a cup of fruit, a dish of olives, plain potatoes in some form, vegetables if they are plain (no inappropriate sauces), and a glass or two of vegetable juice. At an Asian restaurant, I try to order vegetables alone, but no soybeans, peanuts, or other unacceptable products mixed with the vegetables. (I have tried white rice at a single meal, and had no visible reaction; however, I did get a slight reaction in my skin when I tested white rice with the standard re-introduction text: one serving/meal for six meals in a row.)
What do I do if all menu items are unacceptable? If (1) I ask politely, explain that I am on a medical diet ("just vegetables, fruit, and potatoes"), and (2) offer to pay a full dinner price (I usually suggest a few dollars more than the average dinner price), most restaurant cooks will gladly make up a plate with sliced vegetables, fruit, and potatoes in some form (baked, plain, for example). If I can't find anything suitable, I skip that meal, and drink water or tea instead.