Combine all of the ingredients in a heavy-bottomed stock pot and place over medium-high heat; bring the liquid slowly to a boil. When the broth begins to boil, skim any impurities that may rise to the surface, then reduce the heat to a very low simmer. Simmer the broth very slowly-bubbles should just be breaking the surface-for 3-5 hours. Occasionally skim the foam that rises to the surface. Strain the broth through a fine sieve. Refrigerate or freeze until needed.