Cook pasta according to package directions, drain, and spoon into a serving bowl.
While pasta is cooking, heat oil in a large frying pan over medium heat. Cook onion and garlic until softened, about 5 minutes. Add mushrooms and cook until starting to brown, about 10 minutes. Add tomato paste, diced tomatoes, rosemary, salt, and pepper. Simmer 10 minutes.
Serve pasta topped with sauce and oregano. Serve shaved parmesan on the side.
Note: Nutritional analysis is per 2-cup serving.