How to Make It
Heat the olive oil in a medium saucepan. Add the onion, carrot, celery and sage and cook over low heat until the onion is softened but not browned, about 10 minutes. Add the tomatoes, tomato paste and 1 tablespoon of basil and season with salt and pepper. Cover and cook for 30 minutes.
In a medium saucepan of water, simmer the sausage over moderate heat until cooked through, about 8 minutes. Drain, cut into 1/4-inch rounds and add to the tomato sauce.
Cook the fennel slices in a large pot of boiling salted water until just tender, about 3 minutes. Using a slotted spoon, transfer them to the tomato sauce. Add the penne to the boiling water and cook, stirring occasionally, until al dente; drain and return to the pot. Add the tomato sauce, parsley and the fennel fronds and toss. Add the 2 tablespoons of cheese and the remaining 1 tablespoon of basil and toss again. Transfer the pasta to a warmed bowl and serve with the additional cheese.