In a large skillet, cook the beef and onions over medium heat until no longer pink. Add the garlic and cook for 1 minute longer. Drain off any grease.
Stir in the spaghetti sauce, diced tomatoes, mushrooms and Italian seasoning. Bring the mixture to a boil. Reduce heat and simmer, uncovered, 20 minutes.
Meanwhile, preheat the oven to 350°F. Cook the pasta according to package directions; drain.
Add the cooked pasta to the beef mixture and gently stir in the sliced plum tomatoes.
Transfer to an ungreased 13x9-in. baking dish. Sprinkle with cheeses. Bake for 25 to 30 minutes or until bubbly and heated through.
