Bring large pot of water to a boil. When water boils, add salt and pasta, stirring to prevent sticking. Cook pasta according to package directions.
Meanwhile, in large saucepan over medium heat, sauté onion, celery and mushrooms and cook, stirring often until beginning to soften, about 5 minutes. Add tomatoes, tomato puree, oregano, parsley, salt and pepper. Cook over medium heat, stirring occasionally, 5 to 7 minutes, until vegetables are tender. Stir in red pepper flakes to taste.
Drain and rinse pasta under cold running water; drain again. Add to vegetable mixture and stir until well coated and thoroughly heated.