1. In a large skillet, heat 2 tablespoon (25 ml) of the olive oil over medium-high heat and cook onions until tender puree in food processor, onions and set aside.
2. In a large pot of boiling salted water, cook penne 8 to 10 minutes such that al dente.
3. To prepare the sauce, in a large saucepan, heat remaining olive oil over medium-high heat, cook mushrooms, garlic and rosemary about 5 minutes such that tender and stir in onion puree.
4. Toss drained pasta with sauce and Parmesan cheese and serve.