Steps
- Pulse 2½ cups of all-purpose flour and 1 teaspoon of table salt. Add 1 cup of cold cubed unsalted butter and pulse to pea-size pieces, then add ¼ cup of cold water and pulse to a shaggy dough.
- Flatten into a 1in / 2.5cm thick disk, wrap, and chill for 1 hour. Preheat the oven to 350°F / 175°C.
- Roll on a floured surface into a 14in / 35cm circle. Line a 9in / 23cm pie pan at least 1½in / 4cm deep, chill for 10 minutes, and prick the base.
- Line with parchment and pie weights, place on a rimmed baking sheet, and bake for 20-25 minutes. Remove the paper and weights and bake on the bottom rack for 20-25 minutes, until golden.
- Whisk 1 cup of mayonnaise, ½ cup of buttermilk, and 2 large eggs with two-thirds of ¾ cup of freshly grated Parmesan. Add 6 finely chopped scallions, white and light green parts and ¼ teaspoon of black pepper.
- Pour three-quarters of the custard into the crust. Arrange most of 3 medium cored thickly sliced tomatoes on top, overlapping slightly, then scatter with ½ cup of torn basil leaves.
- Add the remaining custard, Parmesan, and tomato slices. Bake for 45-55 minutes, until set and browned, shielding the crust if needed. Cool for 30 minutes.