Heat the butter and olive oil over medium-high heat.
Remove the sausage from its casing, break it into small pieces, and brown well.
Add the onion and cook for about 2 minutes until softened.
Season with salt, black pepper, and a generous pinch of ground fennel seeds.
Pour in the heavy cream and let it simmer until slightly reduced.
Cook your pasta until just shy of al dente, then transfer it to the sauce with a splash of pasta water.
Finish cooking the pasta in the sauce until it’s silky and coats every strand.
Top generously with freshly grated Pecorino Romano and enjoy.
