Peel the onions, cut them in half, then into 1/4-inch (7mm) slices (you can also dice the onions).

Heat butter and olive oil in a large, heavy-bottomed pot. When hot, add the onions and 1 teaspoon of salt.

Cook the onions for about 45-50 minutes until caramelized – they will be soft and light brown/golden in color.

At the beginning you can stir the onions only from time to time but as they darken you will need to stir more often. While stirring make sure to scrape all the brown bits on the bottom of the pot. Be careful not to burn the onions – they will be bitter. Anytime the onions look like they are starting to burn on the bottom of the pot – stir them with a spatula or deglaze the pot with a small amount of water.

The heat will depend on the stovetop and it can vary. Start cooking the onions on medium or medium-high heat (to evaporate excess liquid from the onions), then reduce to medium/medium-low, then cook the onions on low at the end of the cooking time. Adjust the heat to lower if you see that the onions are browning too fast, and increase the heat if you see that the onions are cooking/browning too slow.