Slice the onions thinly, aiming for uniform pieces about 1/8 inch thick.

In a large skillet over medium heat, melt the butter or heat the olive oil until hot but not smoking.

Add the sliced onions to the skillet and stir gently with a wooden spoon.

Sprinkle in the salt and, if desired, add sugar to enhance sweetness.

Cook on medium-low heat for 30–40 minutes, stirring occasionally until the onions are beautifully golden brown and caramelized.

Remove from heat and use immediately or store in an airtight container in the fridge for up to five days.