Bring a large pot of lightly salted water to a boil. Add pasta and cook for 8 to 10 minutes or until al dente; drain.
Drain diced tomatoes, reserving 2 tablespoons of the liquid. Microwave onion, garlic and mushrooms, and reserved liquid in a large, loosely covered dish for 2 to 3 minutes until onions are clear.
Add tomato sauce, Italian seasoning and tomatoes to onion mixture and microwave 5 to 6 minutes more or until heated through. Pour over cooked pasta.
