Stir together first 3 ingredients. Spoon 2 tablespoonfuls of mixture into each shell. Arrange shells, stuffed side up, into a lightly greased 11- x 7-inch baking dish. Set aside.
Cook beef, onion, and garlic in a large skillet over medium heat 15 minutes, stirring until beef crumbles and is no longer pink; drain.
Stir in tomatoes and next 4 ingredients; bring to a boil. Cover, reduce heat, and simmer, stirring occasionally, 10 minutes. Stir in tomato sauce; cook 2 minutes. Pour beef mixture over shells.
Bake at 350° for 15 minutes. Sprinkle with Parmesan cheese, if desired.