Spoon the olive oil into a large, heavy-bottomed sauce pan, preferably cast iron. Add sausage (if sausage is in casings squeeze the meat out). Over low to medium heat, break sausage into small pieces with a wooden spatula and cook for 10 minutes or so, tossing frequently, until nicely browned but not burned. Add onion and cook until translucent. Add garlic and stir with onions and sausage until golden. Add tomatoes and mix thoroughly with other ingredients. Use the spatula to break the tomatoes apart. Cook over a low flame, partially covered, for 15-20 minutes until the tomatoes have softened. Remove cover. Break tomatoes apart more with spatula if needed. Simmer 10-15 minutes more until tomatoes have cooked down and sauce has thickened. Stir in parsley. Toss with penne, fusilli or your favorite pasta and top with grated Parmesan. Serves 4.