Roast the lamb head for 30 minutes in an oven preheated at 375F. Let it cool.
Add lamb head, vegetables and the apple cider vinegar to the slow cooker and add water to cover the bones by about an inch.
Let the mixture sit for 30-60 minutes without turning on the heat. Leaving the mixture sit while cool will permit the vinegar to extract the minerals from the bones.
Simmer in the slow cooker on low for 24 to 36 hours. Top with water if too much evaporates.
Strain the broth and salt to taste (I only used 1/2 tsp). Let it cool in the fridge and skim the fat off the top once hardened. Your broth is ready.