Fill a pre-baked pie crust with tomatoes and sweet onions, in a 2 - 1 ratio. You can alternate thin slices or use store bought chopped stuff. Salt & pepper to taste. My Wife added a bit of Basil.
Then you take shredded cheddar cheese and mix it with mayonnaise and one teaspoon of mustard. A recipe I saw on line said two cups of cheese and one cup of mayonnaise. Then you spread the mix on top of the tomatoes, covering all of the way to the edges.
Then you bake it at 350 degrees for 30 minutes. The onions were not fully cooked so next time I will cook it for 40 minutes.
The cheese/mayo mix melts and goes down into and through the tomato/onion mix.