Melt the butter in a large saucepan and add the chopped tomatoes, garlic and rosemary. Season with salt and pepper and a pinch of sugar. Cook the tomatoes until they have begun to soften, or for about 5 minutes.
Add the sausage to the pan with the chili flakes, cream and half the chopped parsley. Allow to simmer with the lid off until the mixture has reduced by half, 10-20 minutes, stirring frequently. Take off the heat and check and adjust the seasoning.
Meanwhile cook the pasta in a large pan of salted boiling water until it is slightly al dente (8-10 minutes). Once cooked, drain and toss with the sauce.
Plate the pasta in a big serving bowl and scatter with the grated parmesan cheese and the rest of the parsley and serve immediately.