1. In a large saucepan, boil water for tortellini preparation.
2. Meanwhile, preheat olive oil in a large skillet on medium-high heat; sauté onion and garlic for 3 to 4 minutes or until golden brown.
3. Add mushrooms, meatballs and diced tomatoes.
4. Simmer for 15 minutes, stirring occasionally.
5. Add tortellini to boiling water, cook 3 to 3 1/2 minutes; drain.
6. Pour sauce over tortellini; sprinkle with Parmesan cheese.