By hand: In a large bowl, combine the flour with cornmeal, salt, and sugar. Add the butter, and using a pastry cutter, cut the butter into the flour until it resembles coarse sand. Gradually add the water until the dough comes together but not wet.
Using a food processor: Combine the dry ingredients, then add the butter and pulse 10-12 times and the mixture will resemble coarse sand. Gradually add the water (1 tablespoon at a time, you might not need all of the water) until the dough comes together but not too moist (about 1/4 a cup, 60ml).
Over plastic wrap, shape the dough into a flat disk and chill in the fridge for 30 minutes and up to an hour.
Preheat the oven to 400°F (200°C). The galette will be baked on the middle rack.
Between 2 pieces of parchment paper, roll out the dough into a 12-inch circle.
To make the filling, in a bowl combine the sliced strawberries with sugar, lemon juice, and cornstarch.
Place the galette over a baking sheet and add the filling into the center of the pie leaving about 2-2.5 inches border and then fold the edges over the filling. Brush with the egg wash and sprinkle with granulated sugar if desired.
Bake until the pie crust is deep golden brown for about 30-35 minutes. Transfer to a wire rack to cool completely before you brush the fruit with glaze and serve.