PIECRUST
1 1/4 cups all-purpose flour
1/4 cup cold vegetable shortening, cut into pieces
4 tablespoons cold unsalted butter, cut into pieces
1/2 teaspoon fine sea salt
3 to 4 Tbsp. ice-cold water
FILLING
2 1/4 pounds assorted heirloom tomatoes, thinly sliced
1 1/4 teaspoons kosher salt, divided
1 sweet onion, chopped
1 1/4 teaspoons freshly ground pepper, divided
1 tablespoon canola oil
2 tablespoons assorted chopped fresh herbs (such as chives, parsley, and basil)
1/2 cup freshly grated Gruyère cheese
1/2 cup freshly grated Parmigiano-Reggiano cheese
1/4 cup mayonnaise

Piecrust:
Process first 4 ingredients in a food processor until mixture resembles coarse meal.
With processor running, gradually add 3 Tbsp. ice-cold water, 1 Tbsp. at a time,
and process until dough forms a ball and leaves sides of bowl, adding up to 1 Tbsp. more water, if necessary.
Shape dough into a disk, and wrap in plastic wrap.
Chill 30 minutes.
Unwrap dough, and place on a lightly floured surface; sprinkle lightly with flour.
Roll dough to 1/8-inch thickness.
Preheat oven to 425°.
Press dough into a 9-inch pie plate.
Trim dough 1 inch larger than diameter of pie plate; fold overhanging dough under itself along rim of pie plate.
Chill 30 minutes or until firm.
Line piecrust with aluminum foil; fill with pie weights or dried beans.
(This will keep the crust from bubbling up.) Place on an aluminum foil-lined baking sheet.
Bake at 425° for 20 minutes. Remove weights and foil.
Bake 5 minutes or until browned. Cool completely on baking sheet on a wire rack (about 30 minutes).
Reduce oven temperature to 350°.
Filling:
Place tomatoes in a single layer on paper towels; sprinkle with 1 tsp. salt. Let stand 10 minutes.
Meanwhile, sauté onion and 1/4 tsp. each salt and pepper in hot oil in a skillet over medium heat 3 minutes or until onion is tender.
Pat tomatoes dry with a paper towel.
Layer tomatoes, onion, and herbs in prepared crust, seasoning each layer with pepper (1 tsp. total).
Stir together cheeses and mayonnaise; spread over pie.
Bake at 350° for 30 minutes or until lightly browned, shielding edges with foil to prevent excessive browning.
Serve hot, warm, or at room temperature.