How to Make It
Prepare the Crust: Pulse flour, thyme, and salt in a food processor until combined, 2 to 3 times. Scatter diced butter and shortening over top; pulse until mixture resembles coarse sand, 4 to 5 times. Drizzle with ice water; pulse just until mixture starts to clump together, 3 to 5 times.
Transfer dough to a work surface. Gather dough together, and knead until dough comes together, 3 to 4 times. Shape dough into a disc. Cover with plastic wrap, and chill 30 minutes.
Prepare the Filling: Heat butter and oil in a large skillet over low. Add onion, and cook, stirring just enough to keep from sticking or scorching, until golden brown and tender, 20 to 25 minutes. (The more you stir, the more onions will break down.)
Preheat oven to 375°F. Stir together ricotta, zest, and 1/4 teaspoon of the salt in a small bowl.
Remove dough from refrigerator; remove and discard plastic wrap. Place dough on a lightly floured piece of parchment paper, and roll dough into a 14-inch circle. Spread ricotta mixture evenly over dough, leaving a 2-inch border. Arrange spiralized zucchini and squash over ricotta mixture. Sprinkle evenly with thyme, pepper, and remaining 3/4 teaspoon salt. Top evenly with caramelized onions and tomatoes. Fold edges of dough up and over filling to partially cover.
Whisk together egg and water in a small bowl. Brush egg mixture over exposed dough. Place galette and parchment on a baking sheet. Bake in preheated oven until well browned, 45 to 50 minutes. Cut into 6 wedges.