In a saucepan, heat the oil and brown the sausages evenly. Remove the sausages; cut them into thin slices. Pour the oil out of the pan and discard it, but leave the brown residue. Add the tomato sauce to the pan pan; add the sausage slices. Stir, cover and simmer for 20 minutes. Butter a baking dish and spoon a layer of the boiled, drained ditali evenly across the bottom; cover with a layer of sausage slices in the tomato sauce. Then spread with a layer of ricotta and sprinkle it with Parmesan cheese. Repeat the layering until all of the pasta, sausages and cheese are used, ending with a layer of sauce and cheese. Bake uncovered, in a preheated 350 degree oven for 25 minutes, or until the sauce is bubbling. Serve in warmed individual plates. Serves 6