Preheat your oven to 375°F (190°C). Place a baking sheet in the oven to preheat as well.

If using a homemade pie crust, roll it out and fit it into a 9-inch pie dish. Crimp the edges decoratively and refrigerate while preparing the filling.

In a large bowl, gently toss the blueberries with sugar, cornstarch, lemon juice, lemon zest, cinnamon, nutmeg, and salt until evenly coated.

Pour the blueberry mixture into the prepared pie crust, arranging it in an even layer.

For the crumble topping, combine flour, brown sugar, oats, cinnamon, and salt in a medium bowl. Add the cold cubed butter and use a pastry cutter or your fingertips to work the butter into the dry ingredients until the mixture resembles coarse crumbs. Stir in chopped pecans if using.

Sprinkle the crumble topping evenly over the blueberry filling, covering it completely.

Place the pie on the preheated baking sheet (this catches any drips) and bake for 20 minutes at 375°F.

Reduce the oven temperature to 350°F (175°C) and continue baking for 30-35 minutes more, until the filling is bubbling and the topping is golden brown. If the edges begin to brown too quickly, cover them with aluminum foil.

Remove from the oven and cool on a wire rack for at least 2 hours before serving to allow the filling to set.