Remove sausages from their casing, heat a tablespoon of olive oil in a deep pan and cook sausage meat over medium heat while breaking it up in small chunks with a wooden spoon until no longer pink inside for 7-10 minutes. Add the garlic, fennel seeds and cook for 1-2 minutes.
Add canned tomatoes and tomato passata (or crushed tomatoes) along with the chilli flakes and fresh basil leaves, bring to a boil and then turn the heat down and simmer for 10-15 minutes. Season with salt to taste.
Meanwhile cook the pasta according to package directions. Drain the pasta and add to the sauce, toss to coat. Serve with dollops of ricotta cheese.