In large pot of boiling water, cook noodles according to package directions or until al dente. Drain and rinse under cold running water; drain and set aside.
In bowl, combine beef, breadcrumbs, egg and thyme. Mix well and shape into 1” balls.
In heavy Dutch oven or flameproof casserole, heat oil over medium heat; cook meatballs, in single layer and in batches if necessary, stirring occasionally for about 10 minutes or until meat is no longer pink. Drain off excess fat.
Stir in onions, cook for 2 minutes. Stir in garlic, pepper, mushrooms, celery, oregano and basil; cook for 2 minutes. Add tomato sauce, parsley, noodles and half of the mozzarella cheese; stir gently to mix. Sprinkle with parmesan and remaining mozzarella. (Casserole can be prepared to this point, cooled, covered and refrigerated for up to 2 days or frozen for up to 3 months).
Bake, covered in 350ºF oven for 20 minutes; uncover and bake for 10-15 minutes longer or until heated through and bubbly.
