Heat the skillet over medium heat and add the butter.

Once the butter is melted, add the sliced onions and stir to coat.

Sprinkle salt and sugar (if using) over the onions.

Cook the onions, stirring occasionally, for about 30 minutes until they are golden brown and caramelized.

If the onions start to stick, add a splash of water to deglaze the pan.

Remove from heat and serve or store for later use.