1. Put all ingredients in a large pot and add 4 qts. water. Bring to a boil, then reduce heat and simmer until broth is reduced by half, 2 1/2 to 3 hours; at the beginning, spoon off any foam that forms on top.
2. Pour mixture through a strainer into a bowl and let broth cool; discard vegetables.
3. Use broth, or ladle it into qt.-size freezer bags or containers (great for soup) or ice cube trays (handy for sauces) and freeze up to 6 months; transfer cubes to bags once they're solid.