In a large skillet, melt the butter in the olive oil over medium-high heat. Add the onions and 1/4 teaspoon each salt and pepper, and cook, stirring constantly, until the onions begin to soften, about 5 minutes.
Stir in the sugar and cook, scraping the browned bits off the bottom of the pan frequently, until the onions are golden brown, about 20 minutes.
