Both the pie crust and filling can be made ahead of time! However, I would not store the galette WITH the filling. This is because the crust will absorb the juices from the filling as it sits in the fridge.  Roll out the pie dough and cover it with plastic wrap and store it in the fridge for up to 2 days. The filling can also be mixed and stored in the fridge for up to 2 days in an air-tight container.  Since the prep work has already been done, just assemble the galette and bake on the day you need it.  Galette is best eaten the same day it's made.