Cook macaroni according to package directions. Meanwhile, in a large Dutch oven, cook the beef, onions, carrots and celery over medium heat until meat is no longer pink; drain. Add the tomatoes, salt, garlic powder, pepper and oregano. Bring to a boil. Reduce heat; cover and simmer for 30 minutes or until vegetables are tender.
Drain macaroni; add macaroni and spinach to beef mixture. Pour into two greased 3-qt. baking dishes. Sprinkle with cheese. Bake, uncovered, at 350° for 25-30 minutes or until heated through. Yield: 2 casseroles (12 servings each).