Place the onion, carrot, celery, garlic, and rosemary in a processor and process until finely minced. Remove the sausage meat from the casing and crumble the meat; discard the skins.
Place a large casserole pot over medium heat. Add a splash of olive oil to the pan. Add the sausage meat to the pan and saute until golden brown.
Add the chopped vegetable mixture and the chilli flakes, fennel seeds, bay leaf, and tomato paste and saute for 15 minutes to soften and caramelise. Next, add the tomatoes, stock, vinegar & sugar and bring to a simmer. Cook for 45 minutes on low heat until the sauce becomes rich and thick. Taste and adjust seasoning to suit.
Cook pasta according to instructions, then return cooked pasta to its cooking pot. Add enough sauce to liberally coat the pasta, using some of the reserved pasta water to help the sauce cling to the pasta. Heat through for a minute or so. Spoon into serving dishes and garnish with grated parmesan.