Preheat oven to 350°F.

In a medium bowl, combine mozzarella cheese, Cheddar cheese, and mayonnaise; mix well. Spread half the mixture into baked pie shell; set aside.

Meanwhile, in a medium skillet, heat oil over medium heat and sauté onion slices 4 to 5 minutes, until translucent and soft.

Place onions and tomatoes over cheese mixture. Sprinkle with bacon pieces, reserving a few for garnish. Spread remaining cheese mixture on top and sprinkle with reserved bacon pieces.

Bake 30 to 35 minutes, or until golden. Let cool 5 minutes then slice and serve.