1. 6 servings
2. Preheat the oven to 450 degrees. Place a large heavy baking sheet on the bottom of the oven. Lay the pizza crust on a work surface.
3. Spread the crust with the sauce, followed by the spinach and onion. Sprinkle with the mozzarella and mushrooms. Dollop the ricotta over the pizza and sprinkle with the Parmesan. Drizzle the olive oil all over.
4. Use a thin cutting board or the back of a second baking sheet to carefully transfer the pizza to the preheated baking sheet on the bottom of the oven. Bake until the crust is puffed and beginning to color at the edges and the cheese is melted, 10 minutes. Take the sheet out of the oven and preheat the broiler.
5. Broil the pizza, watching carefully, until the cheese is browned and bubbling, about 2 minutes. Let pizza cool on the sheet for 5 minutes. Sprinkle with the balsamic vinegar, slice into wedges, and serve.