Heat a large sauté pan over medium-low. Add 3 tbsp. of butter, vegetable or olive oil to the pan, then add at least 4 c. of thinly sliced onions—they’ll shrink down significantly during caramelizing.
You can caramelize any color of onion you’d like—red, yellow or white onions work equally well. Or use a combination of colors.
Stir the onions to coat with fat, then spread out evenly in the pan. Lightly salt the onions—this helps to draw out moisture—and cook them slowly over medium-low heat, stirring occasionally.
Avoid stirring the onions too frequently. Prolonged contact with the surface of the pan will help promote caramelization.
Cook the onions until they are very soft and have turned a golden brown color. Don’t rush the process—low and slow is critical to properly caramelize the onions. It may take over 30 min. to reach the color you want.
Store caramelized onions in a plastic container in the refrigerator for up to 1 week. You can also freeze them for up to 1 month.