PENNE WITH CARAMELIZED ONIONS & RADICCHIO

16 ounces dried or fresh penne pasta 
salt 
1 head radicchio 
2 teaspoons olive oil 
1 large onion, thinly sliced 
1 tablespoon balsamic vinegar 
1/4 teaspoon coarsely ground black pepper
1 cup fresh or frozen peas (thawed) - optional
1/4 cup crumbled ricotta salata or goat cheese

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" slices. Set aside. In a 12" 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.