Preheat the oven to 350°F (180°C).
Parboil the onions in boiling salted water for 15 minutes.
Melt the butter or bacon fat in a heavy casserole.
Add the onions and the sugar and cook over low heat, turning the onions occasionally, for 5 to 10 minutes.
Season well and put the casserole into the oven for 1 hour or until the onions are tender and a rich gold color.
Baste from time to time, and if the onions get too dry pour in a little stock.