1½ pounds
yellow onions (about 3 large), halved and cut into ¼-inch-thick slices
Fine sea salt
1 teaspoon
light brown sugar (optional)
Method
In a large nonstick skillet, heat the butter and olive oil together over medium heat. Once the butter is melted, add the onions and cook, stirring often, until softened and translucent, about 10 minutes. Reduce the heat to medium-low, spread the onions into a single layer, and cook, stirring every 10 to 15 minutes, then returning the onions to a single layer, until the onions are golden brown and caramelized, 45 to 60 minutes total. If the onions are browning too quickly, add water, 1 tablespoon at a time, until they stop browning and return to caramelizing.
Taste and add salt as needed. Stir in the brown sugar, if desired. Store refrigerated in an airtight container for up to 1 week.