Preheat your oven to 400°F (200°C). Roll out the pie crust and press into a 9-inch pie dish.
In a large bowl, toss the sliced apples with lemon juice, granulated sugar, brown sugar, flour, cinnamon, nutmeg, and sea salt until evenly coated. Transfer apples to the prepared crust.
Prepare the crumble by combining the rolled oats, flour, and brown sugar in a bowl. Cut in the cold butter until coarse clusters form.
Pile the crumble evenly over the apple filling.
Bake at 400°F for 20 minutes, then reduce oven temperature to 375°F (190°C) and bake for another 30 to 35 minutes until the apples are tender and the topping is golden brown.
Sprinkle lightly with coarse sea salt flakes before serving warm.