In a small bowl, whisk together the flour, sugar, and salt. Transfer to a food processor fitted with the dough blade.

Add the cold cubed butter to the flour mixture. Pulse until the butter pieces are about the size of peas and beans—some variation is fine, but none smaller than a pea.

With the food processor running, drizzle the cold water through the feed tube. Stop as soon as the dough starts to come together. Use a spatula to scrape any flour from the bottom if needed.

Turn the dough out onto a lightly floured surface and shape into a disk. Wrap in plastic and refrigerate for at least 1 hour.

Repeat the same steps to make the second dough disk for the top crust.

Wash and hull the strawberries, then cut into quarters (or eighths if very large). Wash the blueberries.

In a large bowl, toss the berries with 1/4 cup of the turbinado sugar and the lemon juice. Let macerate at room temperature for 1 hour.

After maceration, strain the berries and discard the liquid. In the same bowl, combine the remaining 3/4 cup turbinado sugar, lemon zest, and cornstarch. Add the drained berries and mix well.

Remove one dough disk from the fridge. On a lightly floured surface, roll it into a circle about 2 inches larger than your pie dish. Use extra flour to prevent sticking.

Carefully transfer the dough to the pie dish, gently pressing it into the bottom and up the sides. Leave the overhang in place.

Trim the excess dough to create an even edge, leaving enough to fold under and flute the edge.

Cover the prepared bottom crust with plastic wrap and place it in the freezer (or fridge) until needed.

Roll out the second dough disk to about 1/8 to 1/4 inch thick. Cut out about 20 star shapes (or desired shape) using a 3-inch cutter. Place the stars on a plate or baking sheet and freeze (or refrigerate) until firm.

Preheat the oven to 400°F.

Remove the bottom crust from the freezer. Prick the bottom and sides thoroughly with a fork.

Separate the egg; beat the egg white with a fork. Brush the egg white over the bottom crust to create a moisture barrier.

Fill the crust with the prepared berry mixture. Arrange the frozen star shapes on top to form a lattice-like top crust.

Add the egg yolk back to the remaining egg white along with the cream or milk; beat together to make the egg wash. Brush the egg wash generously over the top crust and sprinkle with turbinado sugar.

Tear long strips of foil and wrap them around the pie dish to shield the outer crust. Bake for 40 minutes with the shield, then remove the foil and bake for another 10–15 minutes, until the crust is deep golden and the filling bubbles.

Remove from the oven and let the pie cool completely on a wire rack before slicing.