Combine sugar, cornstarch, and a big squeeze of lemon juice with the strawberries and the blueberries, separately.
Roll one of the pie crusts onto a baking sheet lined with parchment paper.
Add the strawberries in a ring around the dough, leaving a 1.5 inch edge for folding later.
Add the blueberries to the middle.
Fold the edge of the pie dough up around the fruit. Pinch the dough together every 3 inches or so.
Brush the dough with milk, heavy cream, or egg wash.
Pop it in the oven for 25 minutes. Ovens vary so check your galette to make sure it's not browning too fast. If it is then turn down your oven by about 20 degrees and continue to keep an eye on it.
Take the galette out. It should be a deep golden brown. Let rest for 10 minutes. Cut and serve with vanilla ice cream or whipped cream.