Once onions are cooked to medium/well, add can of tuna (which has been drained).
Cook for a minute or two, stirring.
Add canned chopped tomatoes, stir and let simmer on a very low setting for about 5 minutes while you finish mac and cheese. Do not overcook as you'll lose the moisture and you don't want that to happen. Cover pan if you like.
Prepare mac and cheese according to box directions.
Place completed macaroni & cheese along with tuna mixture into a bowl, add salt and pepper (if desired) and combine.