In a large frying pan, heat the oil over moderate heat. Add the sausage and cook, breaking up the meat with a fork, until it is no longer pink, about 5 minutes. With a slotted spoon, remove the sausage from the pan. Discard all but 1 tablespoon fat.
Reduce the heat to moderately low. Add the onion and garlic and cook, stirring occasionally, until the onion is translucent, about 5 minutes. Add the stock and cook until it almost evaporates, about 5 minutes. Stir in the sausage, Marinara, and salt. Simmer, covered, for 10 minutes. Add the pepper, parsley, and cream.
In a large pot of boiling, salted water, cook the rigatoni until just done, about 14 minutes. Drain the pasta and toss with the sauce. Serve with grated Parmesan.