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.
For the glaze
Mix the jam with the water then carefully brush the glaze over the strawberries (the filling will be soft, so do it carefully).
Serve the galette with vanilla ice cream, whipped cream, whipped mascarpone cream, or on its own.
Make sure that the dough is not too soft/moist when making it. You might not need to add all of the water, so add it gradually about 1 tablespoon at a time. The dough will be a bit too crumbly when you roll it out, and that's how it should be.
Roll out the dough into a 12-inch circle. If you roll out a smaller circle, the dough will be too thick and chewy when baked. You want a nice and flaky crust!
Do not overbake the galette, unlike a pie the galette is rolled out into a thin layer so it will cook much quicker.
You can make the dough ahead, and keep it in the fridge for up to 3 days. Remove from the fridge 15 minutes before you start working with it.
Freezing: You can freeze the galette unbaked, but thaw it overnight in the fridge before baking and serving. If you freeze it baked, just thaw it overnight in the fridge, remove from the fridge so it comes to room temperature before serving.