1. Saute the sausage in butter and olive oil until the meat is brown, about 15 minutes. Add the mushrooms and cook over low heat for 10 minutes. Stir in the garlic and peas, and salt and pepper to taste.
2. Cook the penne until al dente. Drain and return to the pot in which it cooked. Add half the cheese and half the sauce. Toss well, then transfer to a warmed bowl. Add the remaining sauce and serve. Pass remaining cheese.