In a medium bowl, toss together 1 pint blueberries, 2 sliced peaches, 1/4 cup cornstarch, 3 tablespoons granulated sugar, and 1 1/2 teaspoons lavender extract until well combined.
Use a spoon to transfer the pie filling into the cold pie crust base, spreading the filling evenly across the base. It's important that you make the filling to be as even and flat as possible — the more even it is, the prettier your lattice will be.
Once you've spread and flattened out the fruit filling, use the chilled strips to weave a lattice top (check out this awesome Food52 video for visual instructions!). Use kitchen shears to shorten and fit the strips onto the pie, before using a fork to crimp the base and the strips onto the base.
Cover loosely with plastic wrap and transfer to the freezer to chill for 1 hour before baking.