Make sure to cook this to the point where they oxtails are falling off the bone. This dish is wonderful as is or spooned over some cauliflower puree or keto-friendly pasta substitutes.
Add everything to the cooker and cook on low for 8-10 hours. Chill overnight then skim the fat and transfer the stew to a large pot or pan to reheat. When warm, remove the oxtails and set aside. Bring the sauce to a boil and reduce to desired consistency - it should be slightly thickened. Season to taste with salt and pepper. Add back the oxtails and warm through before serving.
Notes
I like to cook this dish a day ahead. Chilling the whole stew makes it easy to remove the excess fat, which will be a solid layer at the surface. The meat and sauce will still be plenty rich even after skimming.To cook this on the stovetop:Preheat the oven to 425ºF and oil a rimmed baking sheet with a couple of tablespoons of avocado oil. Add the onion and oxtails, season with salt and pepper, and add another tablespoon or two of oil and toss to coat well. Spread into an even layer and roast for 25-30 minutes, flip the oxtails and cook another 25-30 minutes until browned.Transfer the onions and oxtails to a large heavy-bottomed stock pot or dutch oven. Add all remaining ingredients and cook over medium high heat until boiling. Then, reduce the heat to low, cover, and simmer for 5 hours, or until the meat is easily pulled off the bone. OR return to a 250ºF oven and cook, covered for 3-5 hours, until tender. Either skim excess fat or refrigerate overnight. Remove the oxtails and reduce the sauce to a thicker glaze, making sure to rewarm the oxtails in it before serving.