Cook and stir the meat and the first 1/4 cup of onion in a large skillet until the meat is brown. Drain off excess fat. Remove from the heat and stir in the remaining ingredients for the Meat Filling. Fill the uncooked manicotti shells, packing the filling into both ends. Place the shells in an ungreased baking pan, 13x9". Heat the oven to 375F. Heat the undrained mushrooms and the remaining ingredients for the Tomato sauce except the Parmesan Cheese to boiling, stirring occasionally. Reduce the heat and simmer, uncovered, for about 5 minutes. Pour the sauce over the filled shells. Cover the pan with aluminum foil and bake until the shells are tender, 1-1/2 to 1-3/4 hours. Sprinkle with the cheese and cool 5-10 minutes before serving.
