Preheat the slow cooker if necessary; see the manufacturer's instructions. Put the pasta into a large bowl, cover with boiling water and leave to stand for 10 minutes while preparing the rest of the dish.
Heat 1 tablespoon of the oil in a large frying pan, add the onion and fry until softened. Stir in the remaining oil, garlic and mushrooms and fry, stirring, until the mushrooms are just beginning to brown.
Stir in the fresh and canned tomatoes, stock or wine and tomato purée. Add the rosemary and a little salt and pepper and bring to the boil.
Drain the pasta and put it in the slow cooker pot, pour over the hot mushroom mixture and spread into an even layer. Cover with the lid and cook on low for 2½–3 hours or until the pasta is just tender. Spoon into shallow bowls and sprinkle with grated Parmesan. Serve with a rocket salad.