Make the dough: Combine flour, salt, and sugar in a large bowl. Cut half of the butter into the flour mixture with a fork or your hands until it resembles cornmeal. Add the other half of the butter and work into the dough for 1 minute or so, leaving marble-size chunks.
Slowly add the ice water while mixing the dough with a fork until all of the flour is moistened and the dough is coming together.
Divide and gather the dough into a ball. Wrap in plastic and chill until ready to use. (It can be made up to a day in advance, or frozen for longer storage.) Allow dough to sit out at room temperature for 15–20 minutes so it’s soft enough to roll.
Roll dough into a 10-inch diameter circle and chill while you prepare the filling.
Preheat oven to 350°F.
Lay rolled-out pastry on a parchment-lined baking sheet. Sprinkle ground pistachios or almonds onto dough. Arrange overlapping slices of apple in a circular pattern over the pastry, leaving a 1-inch border at the edges.
Fold dough border over apples, and crimp to seal.
Sprinkle sugar on top of apples and border.
Bake uncovered 45–60 minutes, until pastry is nicely browned and apples are fork-tender and lightly colored. Remove from oven and allow to cool until ready to serve.
Just before serving, melt apricot jam in a small pot or microwave and add 1 tbsp. water to thin. Using a pastry brush, paint the apples with the apricot glaze. Serve warm or at room temperature with lightly whipped cream.