Divide the ingredients between two large pots (or place all of the ingredients in a large stockpot). Fill each pot with enough water to cover the ingredients (about 4 quarts per pot) and bring to a boil.
Lower the heat and simmer until reduced by one-fifth, about 4 1/2 hours. Let cool for at least 2 hours.
Strain the stock through a colander set inside an extra-large bowl; skim and discard any fat from the surface.
Rinse out a pot, then pour in the stock. Bring to a boil over high heat and cook until reduced by half, about 50 minutes.
Refrigerate the stock, then skim and discard any solidified fat. Makes about three quarts.