Heat the oven. Preheat to 400°F (200°C). Place a rack in the lower third to help crisp the bottom crust. Set a foil-lined baking sheet on the rack to catch drips.

Prepare the crust. Fit the chilled pie crust into a 9-inch pie plate. Crimp the edges. Brush with a light egg wash if you like. Keep it in the fridge while you make the filling.

Mix the fruit. In a large bowl, combine apples, cranberries, granulated sugar, brown sugar, flour, cornstarch, cinnamon, nutmeg, salt, vanilla, and lemon juice. Toss until the fruit is evenly coated and glossy.

Make the crumble. In another bowl, whisk flour, oats, brown sugar, cinnamon, and salt. Cut in the cold butter with a pastry cutter or your fingertips until the mixture forms pea-sized clumps. Stir in nuts if using. The mixture should clump when pressed.

Assemble. Spoon the fruit into the chilled crust, mounding it slightly in the center. Sprinkle the crumble evenly over the top, pressing it lightly so it adheres. If using, dust the edges with coarse sugar.

Bake hot, then reduce. Bake at 400°F for 20 minutes. Without opening the door for long, reduce the temperature to 350°F (175°C) and continue baking for 35–45 minutes, until the juices are bubbling thickly around the edges and the topping is deep golden.

Tent if needed. If the crumble or crust edges brown too fast, tent loosely with foil during the last 20 minutes.

Cool completely. Let the pie cool on a rack for at least 3 hours. This sets the filling so slices hold their shape.

Serve. Slice and serve at room temperature or slightly warm. Great with vanilla ice cream or lightly sweetened whipped cream.