Mix flour, sugar, and salt in a large bowl. Cut in cold butter until crumbly. Add water gradually, forming a dough. Shape into a disc, wrap in plastic, and chill for 1 hour.

Preheat oven to 350°F (177°C). Line 2 baking sheets with parchment. Mix berries, sugar, flour, lemon juice, and zest in a bowl until coated.

Roll dough into a 12-inch (30-cm) circle. Cut out 6 circles using a 4-inch (10-cm) cutter. Re-roll dough if needed. Chill circles on baking sheets for 15–20 minutes.

Brush dough circles with egg wash. Place fruit filling in the center of each, leaving a ½-inch (1-cm) border. Fold edges up around filling, leaving the center exposed. Pinch seams to seal.

Add a butter pat on each galette. Brush edges with egg wash and sprinkle with coarse sugar.

Bake for 30 minutes or until crusts are golden and filling is bubbly. Cool for 10 minutes, then transfer to a wire rack. Serve warm or at room temperature.