Place the egg and water in a small bowl and whisk well.
Heat the oven to ~425 degrees F.
Remove the dough from the refrigerator, place it on a floured flat clean surface and roll it out to about ¼ inch thick. The edges will be a bit ragged, this is fine and fits with the overall preparation of the Galette. Try to keep it basically round in shape.
Place the rolled out dough on a parchment covered cookie sheet. The round will be about 12 inches in diameter. Keep in mind it will have irregular borders and not be completely round, this is what you want, that rustic appearance.
Using as offset cake spreader spread the Ricotta cheese mixture onto the dough. Leave about 1 ½ to 2 inches around the edges. You will fold these over once the filling is placed. Take the zucchini and layer them on top of the cheese.
Take the tomatoes and place them on top of the zucchini.
Sprinkle the onion garlic mixture over the zucchini and tomatoes.
Fold over the edges of the dough onto the filling.
Brush the edges well with the egg wash.
Sprinkle the Parmesan Regiano over the egg wash and filling
Sprinkle the salt over the egg washed dough edges.
Bake for 20-30 minutes until the crust is golden brown and the filling bubbles.
Let sit for about 10 minutes then cut with an pizza knife or wheel, or large chef's knife into wedges.
Serve and enjoy!!