Sprinkle the oxtail with salt and pepper. Heat the oil in a Dutch oven over medium-high heat. Brown the oxtails on both sides, about 4 minutes each side. Remove to a plate and reserve.
Cook the garlic and onions in the pan for 2 minutes. Add the tomato paste, allspice and juniper and cook 2 minutes more. Return the oxtails to the pan and add the stock and enough water to cover. Bring to a boil, then reduce to a simmer and cook for 1 hour, occasionally skimming the fat from the surface.
Add the potatoes and celery root and continue to cook until the meat is tender, another 1 1/2 hours, skimming the fat from the surface.
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