Step 1

Melt butter in a large heavy-bottomed stainless steel or enameled cast iron saucepan over high heat. Add onions and cook, stirring frequently, until bottom of saucepan is glazed in a pale brown fond, about 5 minutes. Add 2 tablespoons water and scrape up fond with wooden spoon.

Step 2

Continue to cook, stirring frequently, until fond has built up again, about 2 minutes longer. Add 2 more tablespoons water and scrape up browned bits. Repeat cooking, adding water, and scraping until onions are completely softened and a deep, dark brown, about 15 minutes total. Season to taste with salt. Remove from saucepan rapidly to prevent burning.