1. Place onions in a bowl of cold water for 10 minutes to loosen skins then peel the papery skins off. (it really does help to soak them).
2. Mince one of the onions and set aside.
3. Heat oil in a heavy skillet, large enough to hold all the onions in one layer, over medium heat.
4. Add prosciutto fat and stir until melted.
5. Add garlic and the one minced onion and cook until soft and translucent, about 8 minutes.
6. Add onions in one layer, add sugar, bay leaf, vinegar, salt and 1 cup of water or enough to just barely cover the onions.
7. Bring to a boil, reduce heat and gently simmer until the onions are tender, about 1 1/2 hours.
8. Allow to cool to room temperature in the pan juices before serving.