Prepare the Crust: Preheat your oven to 375°F / 190°C. Lightly coat a 15x10x1-inch baking pan with nonstick cooking spray. Gently press the pizza dough into the pan, stretching it to fit evenly and building up a slight edge for the crust.
Arrange the cheese slices in a single layer over the dough.
Caramelize the Onions: In a large skillet, heat 2 tablespoons (30 ml / 1 fl oz) of the olive oil over medium-low heat.
Add the sliced onions and a pinch of salt.
Cook, covered, for 13 to 15 minutes, stirring occasionally, until the onions are very soft and tender. Uncover the skillet, increase the heat to medium-high, and continue to cook for 5 to 8 minutes, stirring frequently, until the onions are beautifully golden brown and caramelized.
Remove the onions from the skillet and set aside.
Sauté the Mushrooms: In the same skillet, heat the remaining 1 tablespoon (15 ml / 0.5 fl oz) of olive oil over medium heat.
Add the sliced mushrooms, fresh rosemary, and minced garlic.
Season with salt and pepper.
Cook, stirring occasionally, until the mushrooms have released their liquid and are tender and lightly browned.
Drain any excess liquid from the pan.
Assemble and Bake: Spoon the cooked mushroom mixture evenly over the cheese on the pizza dough.
Top with the caramelized onions, spreading them out.
Bake for 25 to 30 minutes, or until the crust is golden brown and slightly crisp on the bottom.
Garnish and Serve: Remove the pizza from the oven and let it cool in the pan on a wire rack for 5 minutes.
Sprinkle generously with fresh parsley, slice, and serve warm.
Don't rush the onions. The key to their deep, sweet flavor is cooking them low and slow.
Patience is a virtue that will be deliciously rewarded.
Drain the mushrooms well after cooking. This crucial step prevents excess moisture from making the pizza crust soggy.