Step 1

In a large nonstick skillet, heat the butter and olive oil together over medium heat

Step 2

Once the butter is melted, add the onions and cook, stirring often, until softened and translucent, about 10 minutes

Step 3

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

Step 4

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

Step 5

Stir in the brown sugar, if desired

Step 6

Store refrigerated in an airtight container for up to 1 week.