One way to cook oxtails in a slow cooker is to make them Caribbean-style. For this recipe, start with 4 pounds of oxtails. Gather one bunch of scallions, six cloves of garlic, a 3-inch piece of fresh ginger and 1 teaspoon each of allspice and sea salt.
Peel and chop the scallions, garlic and ginger and add to a small bowl with the allspice, sea salt and 1/4 teaspoon of crushed habanero pepper. Rub this spice mix onto the oxtails and marinate them for at least one hour and up to 24 hours.
After the oxtails have marinated in the spices, slice one large onion and place the slices in the bottom of a crock pot, along with six sprigs of fresh Thyme. Put the oxtails on top of the thyme and onions, then pour 2 cups of chicken broth or water into the crock pot over everything. Put the lid on and cook on high for six hours.
After the oxtails are finished cooking, remove them from the slow cooker and place them aside. Remove the thyme sprigs from the crock pot and puree the rest of the liquid. The liquid can be pureed in a blender or food processor, or with an immersion blender. Be careful, as hot liquid expands. After the sauce is pureed, return it to the slow cooker along with the oxtails, keep warm and serve.