I work at a restaurant, so unless I bring lunch, I need to find things that can work for me.
Some days, when I work until 9 or so) I have my protein at lunch. Grilled chicken or fish with no butter or seasonings, with steamed veggies. Sometimes a salad with veggies (bell pepper, mushrooms, jalapenos) sauteed in a little olive oil. Can do that with or without chicken/shrimp/salmon. I bring those individual pacts of WF italian dressing but sometimes the oil the veggies were sauteed in are enough for me.
It might seem easier for me since I work there and it's easy to tell the kitchen what I want, but having been a server too, I've seen many people come in and ask for food to be prepared a certain way (no butter, asking about certain healthy options that might not be on the menu). It might take some extra time for the server to explain it, put the order in, etc.. but it can be done.
And don't feel like you need to explain that you're on a diet, or tell them all about IP, or say it's a medical thing, etc. And it's always nice on the servers end if you maybe tip a little extra for their help in accommodating your needs (sorry, as someone who's worked for tips I can't help but throw that one in there).