Several large beef bones, with or without meat attached (marrow bones are great!)
1 Tbs. apple cider vinegar or lemon juice
Combine all ingredients in a crockpot and fill with water. Let cook on a low setting for 48-72 hours. Strain the bones, using a cheesecloth for a finer broth.
Enjoy warm or let cool to use in other meals. Keeps in the fridge for several days or freeze in quart sized containers indefinitely.