Apple Crumble Pie

Name: Apple Crumble Pie
Ingredients: 1 store-bought or homemade flaky pie crust , thawed if frozen; 2/3 cup packed dark brown sugar; 1/4 cup all-purpose flour, plus more for rolling; 1 1/2 teaspoons ground cinnamon; 3/4 teaspoon kosher salt; 1/2 teaspoon ground nutmeg; 1 medium lemon; 1 teaspoon vanilla extract; 2 1/2 pounds Granny Smith or Pink Lady apples, preferably a combination (5 to 6 medium); Vanilla ice cream, for serving (optional); 3/4 cup all-purpose flour; 1/2 cup packed dark brown sugar; 1 teaspoon ground cinnamon; 1/4 teaspoon kosher salt; 8 tablespoons (1 stick) cold unsalted butter