Spread the zucchini over paper towels, sprinkle with salt and let drain (about 30 minutes).
Meanwhile, melt butter in a large skillet and cook the onion over medium heat until soft and golden brown (about 30 minutes)
Preheat the oven to 400F.
To assemble the galette, roll out the pie crust on a well greased cookie sheet.
Layer half of the feta cheese and 1/3 of the provolone in the center of the dough (leave abotu 2" on each side).
Next, layer half of the onions, then the garlic.
Layer the final layer of zucchini, onion, then the remaining cheese.
Fold the edges of the galette over the filling and brush the edges with egg wash.
Bake the galette until golden brown (30-40 minutes).