Prepare the Oven & Pie Crust
Preheat oven to 375°F (190°C). Place the unbaked pie crust into a 9-inch pie dish and crimp the edges.
Make the Apple Filling
In a large bowl, combine sliced apples, sugar, flour, cinnamon, nutmeg, salt, vanilla, and lemon juice. Toss until apples are evenly coated. Pour mixture into the pie crust and spread evenly.
Make the Streusel Topping
In another bowl, mix flour, brown sugar, cinnamon, and salt. Cut in cold butter with a pastry cutter or fork until crumbly.
Assemble the Pie
Sprinkle the streusel topping evenly over the apple filling, covering completely.
Bake
Place pie on a baking sheet (to catch drips) and bake for 50–60 minutes, or until topping is golden brown and apples are tender.
Cool & Serve
Let cool for at least 2 hours before slicing to allow filling to set. Serve warm or at room temperature.