Heat oil in a large cast-iron or nonstick skillet over medium-high heat. Add onion, and saute 5 minutes, stirring frequently. Continue cooking 10 to 15 minutes or until deep golden brown, stirring frequently. Add radicchio and salt; saute 5 minutes or until wilted. Add garlic; cook 1 minute. Remove from heat; stir in vinegar. Sprinkle with black pepper.
Preheat oven to 500°.
Roll dough into a 12-inch circle on a lightly floured surface. Place pizza dough on a 12-inch pizza pan or baking sheet coated with cooking spray and sprinkled with cornmeal. Crimp edges of pizza dough with fingers to form a rim. Sprinkle mozzarella cheese on crust. Top with radicchio mixture. Drop ricotta by teaspoonfuls onto the radicchio mixture.
Bake pizza at 500° for 15 minutes or until browned. Cut the pizza into 4 wedges.