Makes 1 (14-inch) pizza
Place a pizza stone in the center of the oven and increase temperature to 500 degrees.
Heat 1/2 tablespoon of oil in a large skillet over medium heat. Add the onion, season with salt and pepper and cook until soft and lightly browned, about 7 minutes. Remove to a bowl.
Add another 1/2 tablespoon of oil to the skillet, then add the mushrooms. Season with salt and pepper. When mushrooms release their water, increase the heat so the water evaporates more quickly. Cook until soft and lightly browned, about 6 to 8 minutes. Add to bowl with the onions.
Add a bit more oil to the skillet and heat to medium low. Add the spinach in handfuls, letting it reduce in volume before adding more. Cook until wilted. Season with salt and pepper and set aside.
Sprinkle some cornmeal onto a large sheet of parchment paper, place the dough on top and pat it into an oval. Lightly dust dough and rolling pin with flour and roll out into a round shape, roughly 14 inches wide. Spread about 1 tablespoon of olive oil over the dough and sprinkle more cornmeal around the edges.
Squeeze the soft garlic cloves out of their skins and spread over the dough. Top with the cheese and the onion-mushroom mixture. Lift the spinach out of the skillet with a slotted spatula, give it a squeeze to release as much water as possible, and spread it over the pizza. Transfer the pizza to the oven by lifting the parchment paper and placing the paper directly onto the pizza stone. Bake for 10 minutes or until the crust starts to brown. Slice and serve.