Heat a pan with olive oil and add the onions and let cook down for a few minutes. Then add in the minced garlic, sliced mushrooms. Let them cook and caramelize for 15-20 minutes until nice and soft. When mushrooms and onions are caramelized, take off the heat and set aside.
Preheat oven to 450 degrees F.
On a flat surface with some flour, roll out the pizza dough until thin and then move that to a oiled baking sheet. Drizzle the dough with olive oil and then bake for 5-7 minutes until lightly baked. Take out of the oven and then top shredded mozzarella cheese, the carmelized onions and mushrooms and parmesan cheese.
Place back in the oven and bake for 9-10 minutes until flat bread is crispy and the cheese is melted. Take out and season with black pepper, arugula and more parmesan cheese.