Begin by thinly slicing Roma tomatoes and laying them in a colander. Sprinkle them generously with salt and allow them to drain for about 10 minutes.

In your pre-baked deep dish pie shell, start layering the drained tomato slices in an even pattern. Top them with fresh chopped basil leaves and thinly sliced red onion.

In a mixing bowl, combine shredded mozzarella, cheddar, and mayonnaise. Stir until well combined and creamy, then spread this mixture evenly over the tomato and onion layers.

Preheat your oven to 350°F (175°C) and bake the assembled pie for 30 to 35 minutes until the cheese is browned and bubbling.

Once baked, remove the pie from the oven and let it rest for about 10 to 15 minutes before serving.