In a large stainless steel saucepan, or in 2 large stainless steel or cast iron skillets, melt butter over medium-high heat until foaming. 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 2 hours. If browned onion juices on bottom of pot threaten to burn, add 1 tablespoon (15ml) water, scrape up browned bits, and continue cooking. Season with salt and pepper.