Position an oven rack in the lower-third position of your oven and place a rimmed baking sheet on it. Preheat oven to 400°F (200°C).
Fit the single pie crust into a 9-inch deep-dish pie pan, crimp the decorative edges, and chill in the refrigerator while preparing the fillings.
In a large bowl, toss the sliced apples with 1/3 cup brown sugar, 1/4 cup granulated sugar, 2.5 tbsp flour, 1.5 tsp cinnamon, nutmeg, sea salt, and fresh lemon juice until evenly coated.
In a separate bowl, prepare the streusel by whisking together 1 cup flour, 1/2 cup brown sugar, 1/4 cup granulated sugar, oats, and 1/2 tsp cinnamon. Add the cold cubed butter and cut in using a pastry blender or fingertips until coarse crumbly clumps form. Keep chilled.
Transfer the spiced apples into the chilled pie crust, mounding them high and compacting gently to minimize empty air pockets. Drizzle any pooled juices from the bowl over the fruit.
Scatter the cold brown sugar streusel crumble evenly over the apples, covering the top completely to the edges of the crust.
Place the pie directly on the preheated baking sheet. Bake at 400°F (200°C) for 20 minutes, then reduce oven temperature to 375°F (190°C) and bake for an additional 35 to 40 minutes (55 to 60 minutes total) until the crust is golden and spiced juices bubble lazily around the edges. Tent loosely with foil if the top browns too quickly.
Remove from oven and let cool completely on a wire rack for at least 2 to 3 hours before slicing so the filling sets cleanly. Serve warm à la mode with vanilla ice cream!