Peel and slice the onions thinly.
In a large skillet, heat olive oil and butter over medium heat.
Add the sliced onions to the skillet.
Stir the onions to coat them evenly with oil.
Sprinkle a pinch of salt to help draw out moisture.
Cook the onions, stirring occasionally, for about 10 minutes.
Reduce the heat to low and add sugar, if desired.
Continue cooking for an additional 20-30 minutes, stirring often.
Once they are golden brown and caramelized, remove from heat.
Season with black pepper and additional salt to taste.