Penne with Caramelized Onions and Radicchio

In saucepot, prepare penne as label directs in boiling salted water. Drain penne, reserving 1/4 cup pasta cooking water. Return penne to saucepot. Meanwhile, cut radicchio lengthwise in half. Remove core, then cut crosswise into 1/2-inch slices. Set aside. In nonstick 12-inch skillet, heat olive oil over medium heat until hot. Add onion and cook until browned and soft, about 15 minutes, stirring occasionally. Add vinegar, pepper, and 1/2 teaspoon salt, cook 1 minute longer. Increase heat to medium-high, add radicchio and cook 2 to 3 minutes until wilted. Add onion mixture, peas, and reserved pasta cooking water to penne, toss to mix well. Serve, sprinkled with cheese.