Slice the tomatoes into 1/2-inch thick rounds and place them in a colander. Sprinkle with salt and let sit for 10 minutes—this draws out excess moisture so your pie won't be watery. Pat the tomato slices dry with paper towels, pressing gently to remove all surface moisture. This is crucial for achieving a crispy crust and preventing a soggy bottom.

While the tomatoes drain, thinly slice the onion and finely chop the fresh basil. In a bowl, combine the mayonnaise, shredded mozzarella, sharp cheddar, grated parmesan, garlic powder, and the remaining 1/2 teaspoon salt and black pepper. Mix until well combined and set aside. I like to prepare this topping while waiting for the tomatoes to drain so everything is ready when you need it.

Preheat your oven to 350°F. Once the oven is heating, arrange the drained tomato slices in the pre-baked pie crust, overlapping them slightly. Scatter the sliced onions and chopped basil evenly over the tomatoes, distributing them as you go. This layering technique ensures every bite has balanced flavor and texture throughout the pie.

Spread the cheese mixture from Step 2 evenly over the tomato, onion, and basil layer, making sure to cover all the vegetables. Bake in your preheated 350°F oven for 30 minutes, until the topping is golden brown and bubbly at the edges. I recommend checking around the 25-minute mark—ovens vary, and you want the cheese to be melted and just starting to brown, not overdone.

Remove the pie from the oven and let it rest for 15 minutes before cutting. This resting time allows the layers to set slightly, making slicing cleaner and helping the pie hold its shape better when plated. Slice into wedges and serve warm or at room temperature.