Heat olive oil in heavy saucepan; add sausage and brown.
Add onion and garlic; cook until the onion is translucent.
Add tomato sauce, tomato paste, sugar, crushed fennel seed, and basil; stir and cook over moderate heat for 20 to 30 minutes.
Bring 4 quarts of water to a boil in a large pot and add all the pasta at once; stir well.
When the sauce is ready, put half-and-half in a bowl and add a little of the hot sauce to it to temper the milk. Add the milk to the rest of the sauce. When the rigatoni is cooked al dente, drain and put in large serving bowl. Add the sauce and stir to coat.
Garnish with basil leaves and serve with fresh parmesan cheese.