In a large saute pan or pot, cook the sausage over medium-high heat until the fat is rendered and the sausages start to brown, about 5 minutes. Remove with a slotted spoon and drain on paper towels. Remove all but 2 tablespoons of fat from the pan.
To the fat remaining in the pan, add the onions and garlic and cook, stirring, over medium-high heat until soft, about 3 minutes. Add the fennel seeds, salt, and red pepper, and cook, stirring, until fragrant, 20 to 30 seconds. Add the tomatoes and their juice, the tomato paste, oregano, and cooked sausage and cook, stirring, until thick and fragrant, 20 to 25 minutes. Remove from the heat and adjust the seasoning, to taste.
Meanwhile, in a pot of salted water, cook the pasta until al dente. Drain and toss in the pot with the sauce. Stir in the ricotta salata and basil. Serve immediately and garnish with Parmesan cheese at the table.