Heat the oil and butter in a large heavy based sauté pan over a medium heat.
Add the onion and sweat for 5-6 minutes. Stirring regularly, the onions should be softened at this point.
Remove the sausage meat from their skins and add to the pan and break up. Cook this for 6-7 minutes.
Add the garlic along with the fennel seeds, chilli flakes and oregano, before stirring through the tomatoes. Simmer for 10-15 minutes over a low heat until reduced a little. Season to taste.
Bring a large pot of water to a boil. Season with salt and add the pasta to this. Cook according to the packet instructions. Drain and add to the sausage pasta sauce.
Blitz the pecorino until fine in a food processor.
Serve the pasta in bowls topped with the finely chopped pecorino and some basil leaves.
