If using a pizza stone, place in the bottom rack of cold oven to prevent cracking.
Pre-heat oven to 450 degrees and line a baking sheet with foil. Sprinkle mushrooms on the prepared baking sheet along with garlic, rosemary, olive oil, salt and chili flakes.
Using your hands, toss mushrooms until evenly coated with oil and seasonings. Roast mushrooms for 10-12 minutes or until caramelized; set aside.
Sprinkle corn meal or flour on a pizza peel or piece of parchment paper. Stretch out dough until you get a nice round circle.
Using a spoon, spread pizza sauce all over the center of the dough, leaving 1 inch border for the crust. Sprinkle evenly with cheese then top with roasted mushrooms.
Carefully transfer pizza to the pre-heated stone in the oven (or an oiled baking sheet) and bake until cheese is bubbly and golden brown, 12-14 minutes.
Using a pizza wheel, slice into 8 pieces and serve with fresh arugula and onion slices.