In a large saucepan, saute the onion and garlic in the olive oil. Add mince and continue cooking. Stir in prosciutto and cook for 5 to 6 minutes. Add 1/2 of the butter, chopped tomatoes and tomato purée. Let sauce simmer for 10 minutes. Season with salt and pepper.
Simmer for one hour. Add remaining butter and simmer for an additional half hour.