Boil salted water for the pasta (we use coarse salt)
Remove the sausage from its casing and cook in a nonstick skillet over low to medium heat. While the sausage is cooking use a spatula to break it up into tiny pieces as it cooks so it all becomes crumbled. If the sausage renders a lot of fat then remove the fat from the pan with a spoon.
Once the sausage is cooked through (but not browned or overcooked) add the milk and passata. Stir, and cook for a few minutes on low heat. Once cooked, check taste and season as needed with salt, black pepper and red chili flakes (if desired — see note regarding seasoning). Set cooked sausage aside.
Cook the pasta 1 minute less than indicated on the box.
When the pasta is done (firm but cooked), reserve some pasta water in a separate bowl, then drain the pasta.
Add the sausage mixture to the drained pasta and stir.
Add a bit of olive oil and most of the cheese and stir. Add in some of the warm pasta water (a bit at a time) to make the sauce silky smooth.
Serve in bowls and sprinkle with remaining cheese.