TO MAKE THE FILLING: combine all the filling ingredients and mix very thoroughly. Heat 4 to 6 quarts of salted water to the boil in a large pot; add 2 tablespoons oil to help prevent pasta from sticking. Add half of the manicotti and cook for 6 to 8 minutes, or until not quite al dente. Stir occasionally with a long-handled fork. Hold a strainer over the pot and use a slotted spoon to remove 1 or 2 manicotti at a time from the boiling water, placing them in the strainer. Shake manicotti free of water over the pot. Spread a clean kitchen towel on the kitchen counter. Place the manicotti side by side on the towel, which will absorb all the remaining moisture. (The shells must be completely dry before stuffing.) Repeat until all shells are cooked. Cool. Generously butter a baking dish, 13 x 10 inches. Fill each shell with some of the stuffing, letting a little of the stuffing come out on either end. This is best done with a pastry tube and a large nozzle; otherwisew use a regular teaspoon. Lay the stuffed manicotti side by side in the buttered dish and surround them with any manicotti shells you may not have sufficient stuffing for. Pour half the tomato sauce over the shells and sprinkle with 1/2 cup Parmesan cheese. Bake in a preheated moderate oven for 15 to 20 minutes. Heat remaining tomato sauce to serve separately. Serve with additional Parmesan Cheese.