1/2 teaspoon ground cinnamon
1/4 teaspoon salt
1/2 cup (1 stick) cold, unsalted butter, cut into 1/4-inch pieces
Preheat oven to 400 degrees. If not already frozen, place the prepared pie crust in the freezer for 15 minutes.
Meanwhile, combine the apples, 1/3 cup of sugar, and lemon juice and zest in a large bowl. Mix well and set aside for 10 minutes.
In a small bowl, mix the remaining 1 tablespoon of sugar with the nutmeg and cornstarch. Add the mixture to the apples and mix well. Transfer the apples to the pie shell and use your hands to level them. Bake on the center rack for 30 minutes.
While the pie bakes, make the crumb topping. Combine the flour, oats, brown sugar, cinnamon and salt in a food processor and pulse several times to mix. Scatter the butter over the top and pulse until the mixture resembles fine crumbs.
Empty the crumbs into a large bowl and rub between your fingers until large, buttery crumbs form. Refrigerate until needed.
Remove the pie from the oven and reduce the temperature to 375 degrees. Carefully pour the crumbs on the center of the pie, spreading them over the surface with your hands and lightly tamping them down.
Return the pie to the oven, rotating it front to back from its previous cooking position. Slide a baking sheet onto the rack below the pie to catch any drips. Bake until the top is golden brown and the juices bubble thickly at the edge, 30 to 35 minutes. If needed, cover the pie loosely with tented foil during the final 15 minutes of baking to prevent the top from browning too much.
Transfer the pie to a wire rack and let cool at least one hour before serving.
Serves 8 to 10.