Preheat oven to 350°F.
Lay sliced tomatoes out on a baking sheet lined with paper towels. Sprinkle with salt and allow them to sit at room temperature for 20-30 minutes.*
In a medium bowl, combine mozzarella, cheddar, parmesan, mayonnaise, garlic powder, oregano, salt, pepper, and cayenne pepper.
Scoop 1 cup of the mixture into the pie shell. Spread to cover the bottom of the pie shell.
Layer the tomato slices, basil, and onion on top of the mayo mixture. Season with salt and pepper.
Spread the remaining mayo mixture on top of the tomatoes and bake for 35-40 minutes or until lightly browned.
To serve, cut into slices and serve warm.