Preheat Oven → Set to 350°F (175°C).
Prepare Tomatoes → Slice tomatoes, sprinkle with salt, and let them drain on paper towels for 10–15 minutes to remove excess water.
Layer the Filling → Place tomato slices in the pie crust. Add onion slices and sprinkle with basil, salt, and pepper.
Make Cheese Mixture → In a bowl, mix cheddar, mozzarella, and mayonnaise until smooth.
Spread Topping → Cover the tomato mixture with the cheese-mayo blend, spreading evenly.
Bake → Bake for 30–35 minutes, or until the top is golden and bubbly.
Cool & Serve → Let the pie rest for 5–10 minutes before slicing.