Meanwhile, in 12-inch nonstick skillet, cook sausage and onion over medium-high heat 5 to 7 minutes, stirring occasionally, until sausage is no longer pink and onion is tender; drain. Stir in celery, zucchini, tomato sauce, diced tomatoes and mushrooms. Heat to boiling; reduce heat. Cook 5 to 10 minutes. Remove from heat. Add cooked pasta; stir gently to mix. Pour into baking dish.