In a small sauté pan over low-medium heat, add butter and onions. Cook until golden brown and caramelized, stirring occasionally. This takes about 15-20 minutes. Set aside to cool.
Slice zucchini as thin as possible and then place in a towel lined bowl. Sprinkle with a little salt, set aside, and let sit for 10 minutes.
Preheat oven to 400F and then assemble the galette.
Place unbaked pie dough on a parchment lined baking sheet. Sprinkle the dough with half of the cheese (leaving about 1 inch along the edges), then the caramelized onions and a little fresh thyme. Wrap the zucchini up in the towel and squeeze as much liquid out as possible and then arrange them on top of the onions. Sprinkle on the rest of the cheese.
Fold the edges over the sides and brush with the egg.
If desired, sprinkle with a little everything seasoning to finish.
Bake for 28-30 minutes until edges are browned. Top with a little more fresh thyme and cool for 10 minutes. Cut and enjoy!