Preheat oven to 350 degrees F. Without peeling or peeling away any cloves, cut the top off the bunches of garlic and place with the marrow bones on a baking sheet (with a lip to catch fat drippings). Drizzle with olive oil and sprinkle with sea salt. Bake for 15 minutes.

In the meantime, place the chopped carrot, onion, and chopped in your slow cooker.

Once the bones are done baking, remove from the oven and place them in the slow cooker along with all of the fat and juices that may have dripped off onto the baking sheet and both heads of garlic. Add filtered water to cover (about 8 cups, depending on how big your slow cooker is).

Set your slow cooker to low and let simmer from 24 – 48 hours. The longer the broth is cooked, the more flavorful and richer it gets. 

Once finished simmering, strain and keep the broth and discard the bones and vegetables. Once cooled, broth can be placed in glass jars and sealed and stored in the fridge for up to 1 week, or in the freezer for up to 2 months.