Thinly slice, or cut into chunks, and pulse in a food processor until finely chopped:

Combine in a stockpot with:

Bring to a boil over medium heat, reduce the heat, partially cover, and cook gently for 1 hour and 15 minutes, skimming often. Strain and let cool uncovered, then refrigerate covered. Skim off any fat. Use immediately, store refrigerated for up to 4 days or freeze for up to 6 months.