Step 1

Make the galette dough. Pulse the flour, cornmeal, sugar, and salt together in a food processor fitted with the blade attachment until combined. Add the butter and pulse until the mixture resembles coarse crumbs, 7 to 10 pulses. It's okay if there are a few large pieces of butter remaining. Drizzle in 2 tablespoons of the ice water and pulse until dough is crumbly in texture but holds together when squeezed, about 4 pulses. If the mixture is dry, pulse in up to 2 more tablespoons of ice water, 1 tablespoon at a time.

Step 2

Chill the dough. Turn the dough out onto a piece of plastic wrap. Flatten into a disk and wrap completely in the plastic wrap. Refrigerate for 30 minutes to 1 hour.

Step 3

Heat the oven: Arrange a rack in the middle of the oven and heat to 400°F.

Step 4

Roll the dough. Unwrap the dough and place it on a piece of parchment paper. Cover with a second piece of parchment. Roll the dough into a 12-inch round about 1/8-inch thick. Slide the parchment and the dough onto a baking sheet and remove the top layer of parchment.

Step 5

Make the fruit filling. Toss the sliced fruit, 1/4 cup turbinado sugar, flour, cinnamon, nutmeg, and salt together in a large bowl.

Step 6

Assemble the galette. Pile the fruit filling onto the dough, slightly mounding it in the center and leaving a 2-inch border around the edge. Fold the rim of the dough up and over the edge of the filling, overlapping the dough as you go around and pleating the dough. Distribute the cubes of butter across the top of the fruit. Beat the egg with 1 tablespoon of the water. Brush the crust with the egg wash and sprinkle with more turbinado sugar.

Step 7

Bake the galette. Bake until the crust is a deep golden-brown and the fruit is cooked, 30 to 35 minutes. Transfer the parchment with the galette on it to a wire rack and cool completely.

Step 8

Brush fruit with glaze and serve the galette. Dilute the jam with the 1 teaspoon of water. Brush the fruit with the glaze. Cut the galette into wedges and serve.