Par-bake the pie crust. If using homemade pie dough, preheat the oven to 425ºF. Place the pie dough in a 9-inch pie dish and bake for 7 to 8 minutes, or until lightly browned. Remove from oven and set aside.
If using store-bought pre-baked pie crust, do not bake. Place the pie crust in a 9-inch pie dish.
Prep. Preheat the oven to 350ºF.
Add the fruit. In a bowl, combine the sliced peaches, blueberries, and lemon zest; toss to combine. Arrange the fruit mixture on the bottom of the pie crust.
Make the custard. In your mixer's bowl, combine sweetened condensed milk, eggs, lemon juice, vanilla, and butter; beat until smooth and combined, about 30 seconds. Pour the custard over the fruit mixture and set aside.
Add the streusel. In a small mixing bowl, combine the flour, brown sugar, and chopped pecans; using a pastry blender, cut in the butter until the mixture resembles coarse crumbs. Sprinkle the streusel topping over the custard.
Bake. Bake the pie for 55 to 60 minutes, or until a toothpick inserted in the center comes out clean.
Cool and serve. Remove the pie from the oven. Let cool completely for at least 4 hours, or overnight, before slicing.