You could get a steak or fish - I think these are the best bets because, as mentioned above, you can specify how you would like them cooked. Usually the fish will come with a sauce, so again, as mentioned, ask for it without the sauce, or ask if you can have the sauce served separately.
Most dishes like this come with potatoes of your choice, so just tell the server that instead of the side of potatoes, could you have some extra salad (they'll usually knock a bit off the bill for you too)
Or you could have a jacket potato and salad. These are usually just microwaved, but make sure to ask them not to put any butter or oil on the potato. Get this with cheese and salad and take most of the cheese off.