Heat the butter and olive oil in a skillet large enough to hold the sliced onions over medium high heat.

Stir in the onions and cook for a couple minutes until softened and beginning to change color.

Reduce the heat and continue to cook until the onions are tender.

Sprinkle with sugar, then stir in the wine and vinegar.

Let simmer until most of the liquid has cooked away and the remaining mix is "jammy."

Season to taste with salt and pepper.