Step 1

Process graham crackers, flour, sugar, and salt in a food processor until crackers become crumbs and mixture is combined. Add butter; pulse until mixture resembles coarse sand with some larger pieces remaining. Add ice water by the tablespoonful, pulsing after each addition, until evenly moistened and dough holds together when squeezed (2–4 Tbsp.). Pat into a 1"-thick disk and wrap tightly in parchment paper or plastic. Chill at least 1 hour.Do ahead: Dough can be made 3 days ahead. Keep chilled.

Step 2

Preheat oven to 400°. Toss peaches, blueberries, cornstarch, lemon zest, cinnamon, and ⅓ cup (67 g) sugar in a large bowl.

Step 3

Unwrap dough and roll out on a lightly floured sheet of parchment to a 12" round. Arrange fruit on top, leaving a 1¼" border. Lift dough edges up and over fruit, pleating as needed. Using parchment paper, slide galette onto a baking sheet; chill 10 minutes.

Step 4

Brush top of pastry with egg and sprinkle with sugar. Dot filling with butter. Bake galette until fruit is softened and crust is deep golden brown, 40–50 minutes (some juice may leak out).

Step 5

Serve galette warm or at room temperature with scoops of ice cream.