• Place a baking sheet in the center rack of the oven and preheat to 400°F / 204°C.
• Place a raw, rolled-out pie crust on a 9" or 10" pie plate. Trim and crimp the edges if desired (see my perfect pie crust recipe and video for crimping technique). Set the pie shell aside.

For the filling

• Add the granulated sugar, brown sugar, flour, cinnamon, and salt to a large mixing bowl. Whisk thoroughly to combine.
• Add the sour cream, heavy cream, and egg to a medium mixing bowl. Whisk thoroughly to combine.
• Add the wet mixture to the large bowl with the dry ingredients. Whisk thoroughly until fully mixed.
• Add the chopped apples and stir to combine. Let the apples soak in the cream mixture as you prepare the topping.

For the topping

• Add the flour, sugar, oats, and cinnamon to a medium bowl. Whisk to combine.
• Using a box or hand grater, grate the cold butter over the flour mixture. Using a fork or pastry cutter, cut the grated butter into the flour mixture until crumbly and no pieces remain that are larger than the size of a pea.

Assembling and baking the pie

• Pour the apple filling into the unbaked pie shell. Sprinkle the topping evenly over the filling until fully covered.
• Lightly beat the egg white and cream in a small bowl using a fork. Brush the rim of the pie with egg wash until coated.
• Place the pie in the preheated oven on the hot baking sheet, and bake for 40-50 minutes until the crust is golden brown and the center of the pie is set. It should not jiggle.
• Let the pie fully cool before serving.