1. Place an oven rack on lowest position and preheat to 500°F.
2. On a lightly floured surface, roll dough into a 14-inch round. Generously sprinkle a baking sheet with cornmeal. Place dough on sheet. Top with Fontina, leaving a 1/2-inch border.
3. Bake until cheese has melted and crust is crisp, about 13 minutes.
4. In a large bowl, whisk vinegar, sugar, salt, garlic and oil. Toss in arugula, radicchio and onion; spread on pizza. Top with Parmesan. Cut into 8 slices. Serve.