Thinly slice the onions. Heat a large skillet over medium-low heat and add the butter. Once melted, add in the onions with a good pinch of salt. Stir to combine everything. Cook, stirring often, for 20 to 30 minutes, until the onions caramelize. I love a classic, slowly caramelized onion, but if you’d like to speed up the process, you can add a pinch of sugar while cooking. You can also make these ahead of time and store them sealed in the fridge, submerged in a bit of olive oil.