You sure can. I usually assemble the whole thing an hour before serving and keep it in the fridge until I am ready to bake it. The important thing is to make sure that you are draining the fruit filling to get all the liquid out (because we do not want a soggy dough) before placing it onto the galette dough. Also, keeping it in the fridge and baking in a preheated oven help to produce a flaky galette recipe.