Pulse in a food processor until coarsely chopped:
Transfer to a stockpot and add:
Bring to a simmer over medium heat, then reduce the heat, partially cover, and cook gently for 30 minutes, skimming often. Add:
Simmer for 1 hour more. Strain and let cool uncovered, then refrigerate covered. Skim off any fat when ready to use. The broth will separate, so whisk before using. Store refrigerated for up to 4 days or freeze for up to 6 months.