1 pound prepared pizza dough, preferably whole-wheat
2 tablespoons extra-virgin olive oil
1 medium onion, halved and thinly sliced
2 ounces very thinly sliced prosciutto, cut into thin strips (about 1/2 cup)
1/4 teaspoon crushed red pepper
1 cup shredded fontina or part-skim mozzarella cheese
2 cups packed coarsely chopped arugula
1 cup chopped tomato
Position oven rack in the lowest position; preheat to 450°F. Coat a large baking sheet with cooking spray.
Roll out dough on a lightly floured surface to about the size of the baking sheet. Transfer to the baking sheet. Bake until puffed and lightly crisped on the bottom, 8 to 10 minutes.
Meanwhile, heat oil in a large nonstick skillet over medium heat. Add onion, prosciutto and crushed red pepper and cook, stirring, until the onion is beginning to brown, about 3 minutes.
Spread the onion mixture evenly over the crust and top with cheese. Bake until crispy and golden and the cheese is melted, 8 to 10 minutes. Remove from the oven and top with arugula and tomato.