Bring a large pot of water to a rolling boil. Salt water generously. Add tagliatelle and stir. Cook pasta al dente (firm to the bite), about 7 to 9 minutes.
While pasta is cooking, add olive oil to a large skillet over medium high heat. Add pancetta and sauté until golden. Add onions and sprigs of rosemary, cook until onions are golden, about 2 minutes. Add garlic and chopped radicchio and sauté about 1 minute. Add tomato paste and cook for 1 minute. Add wine to deglaze the pan. Bring to a boil. Add stock. Bring to a simmer. Season the mixture with salt and pepper. Add balsamic vinegar.
Drain pasta when cooked. Add pasta to the radicchio and onion mixture. Toss to combine. Stir in parsley. Sprinkle with cheese. Garnish with sprigs of herbs if desired. Serve. Serves 4 to 6 depending if served as appetizer or main.