Heat the oil in a large skillet or pan over a medium heat. Fry the diced onion for 5-7 minutes, until starting to soften. Remove to a plate and set aside, leaving any oil in the pan.
Squeeze the sausage meat from the casings into the same skillet / pan and cook for 5 minutes over a medium to high heat until golden, breaking the sausage meat up with a wooden spoon.
Add the minced garlic and fennel seeds and cook for a minute, until fragrant. Then add the tomato paste / tomato puree and return the onions to the pan.
Pour in the tomatoes and season with salt and black pepper.
Bring to the boil, then lower the heat and simmer gently for 10-15 minutes, until the sauce has thickened slightly and the sausage meat is cooked.
Meanwhile, bring a large pot of salted water to the boil and cook the pasta according to the packet instructions.
Drain the pasta, reserving a cup of the pasta cooking water, and toss with the sauce, adding a little of the pasta water if the sauce is too thick.
Spoon into bowls, sprinkle with grated parmesan and chopped fresh basil, if using, and serve immediately.