Prep the crust: Preheat oven to 400°F (200°C).
Fit the pie crust into a 9-inch pie dish. Prick the bottom with a fork. Line with parchment and fill with pie weights or dried beans.
Bake 12–15 minutes until edges are lightly golden. Remove weights and parchment, brush the inside with olive oil, and bake 5–7 more minutes. Set aside to cool slightly.
Drain the tomatoes: Lay tomato slices on a sheet pan lined with paper towels.
Sprinkle with 1 teaspoon kosher salt. Let sit 20–30 minutes. Blot tops dry.
This step helps prevent a watery pie.
Mix the topping: In a bowl, stir together mayonnaise, cheddar, mozzarella, Dijon, vinegar or lemon juice, garlic powder, smoked paprika, and a pinch of black pepper. Taste and adjust seasoning.
Layer the filling: Reduce oven to 375°F (190°C). In the par-baked crust, add a single layer of tomatoes.
Season lightly with pepper. Scatter a few onion slices (if using), half the basil, and half the chives. Repeat with another layer of tomatoes and herbs.
Add the creamy layer: Spread the mayo-cheese mixture over the top, all the way to the crust.
Use the back of a spoon to smooth it evenly. It should seal the filling beneath.
Bake: Place the pie on a baking sheet to catch drips. Bake 30–35 minutes, until the top is golden and bubbly and the crust is deep golden brown.
Rest and slice: Let the pie cool at least 20–30 minutes before slicing.
Garnish with extra basil and chives. Serve warm or at room temperature.