Caramelized Onions

In a large stainless steel saucepan, or in 2 large cast iron skillets, melt butter over medium heat. Add onions and cook, stirring occasionally, until softened, about 8 minutes. Lower heat to medium-low and cook, stirring frequently, until onions are very sweet and a rich golden-brown color, 1 to 1 1/2 hours. If they look like they may start to burn, add a 1 tablespoon water, scrape up browned bits, and continue cooking; add water whenever needed. Season with salt and pepper. Caramelized onions can be refrigerated in a sealed container for up to 1 week. Rewarm before serving.