Preheat oven to 350 degrees.
Place tomato slices in a single layer on paper toweling and sprinkle with salt.
In your pre-baked pie shell, layer the tomato slices.
Mix together the onion, basil, cheeses, and mayonnaise. Adjust flavor with salt and pepper. Spread mixture on top of the tomatoes.
Bake for 30 minutes or until lightly browned. Serve warm.