Very delicious, as a side, cook ahead, if desired, and reheat gently at serving time. This recipe serves three or four, but with other sides at dinner, it probably could serve six, since people mostly eat it as a pickle.
Melt the butter and oil in a heavy, 10-inch skillet.
Add the onions, turn the heat to at least medium, maybe medium-high.
Cook the onions, without stirring too much until they become golden on at least one side, maybe both.
Turn the heat down, add the sugar, and cook until it scrapes up the base, and looks caramel. Avoid touching the sides of the pan, during this initial phase, as it may cause the sugar to harden against the sides, and not retrieve.
Add the wine, and a generous pinch of salt, maybe 1/2 teaspoon, and cook, over medium-low heat, until the onions are well browned, and delicious, about 25 minutes.