Preheat oven to 375 F.

To make streusel topping: Mix flour, chopped pecans and light brown sugar. Stir in butter using fingertips until incorporated with small lumps of butter visible. Refrigerate.

To make filling: In large skillet over medium heat, melt butter until amber, stirring often. Set aside to cool. In large bowl, stir sliced apples, brown sugar, cinnamon, nutmeg, salt, vanilla and flour using spatula. Pour brown butter over apples and mix, scraping skillet with rubber spatula to incorporate brown butter bits.

Place frozen pie shell on baking sheet. Add 2 cups apple mixture into shell, gently pressing apple slices for even base. Add remaining apple slices and gently press. Sprinkle streusel topping evenly over apples.

Bake 30 minutes then reduce oven temperature to 350 F and bake 35-45 minutes until topping is golden and apples feel tender when pierced with fork.