Pair with a spinach and vinaigrette salad for a nice lunch or light dinner.
Preheat oven to 350° F.
Place the tomatoes in a colander in the sink in 1 layer. Sprinkle with salt and allow to drain for 10 minutes.
Layer the tomato slices, basil, and onion in pie shell. Season with salt and pepper. Combine the grated cheeses and mayonnaise together. Spread mixture on top of the tomatoes and bake for 30 minutes or until lightly browned.
To serve, cut slices and serve warm.