Preheat Oven and Prepare Crust: Preheat your oven to 400°F (200°C). Place the unbaked pie crust in a 9-inch pie plate. You can either crimp the edges with your fingers or a fork for a decorative finish.

Make the Filling: In a large bowl, combine the sliced peaches and blueberries. In a separate small bowl, whisk together the flour/cornstarch, granulated sugar, cinnamon, and nutmeg. Sprinkle this mixture over the fruit, along with the lemon juice. Gently toss everything together until the fruit is evenly coated. Spoon the filling into the prepared pie crust.

Make the Crumb Topping: In a medium bowl, combine the flour, brown sugar, cinnamon, and rolled oats (if using). Add the cold, cubed butter. Using a pastry blender, a fork, or your fingertips, work the butter into the dry ingredients until the mixture forms coarse, pea-sized crumbs.

Assemble and Bake: Sprinkle the crumb topping evenly over the fruit filling. Place the pie on a baking sheet to catch any bubbling juices. Bake at 400°F (200°C) for 15 minutes. Then, reduce the oven temperature to 375°F (190°C) and continue to bake for another 35-45 minutes, or until the filling is bubbling and the crumb topping is golden brown. If the crust edges begin to brown too quickly, you can cover them with aluminum foil strips.

Cooling: This is a crucial step for a clean slice. Remove the pie from the oven and place it on a wire rack. Let it cool for at least 2 hours before slicing and serving. The filling needs this time to set properly.