Slice the tail crosswise into portions. The thickest pieces will be used for the stew; save the thinner pieces to the broth. Heat 2 tablespoons of olive oil in a skillet and when hot, add the tail portions and brown on all sides. In a separate pot, boil the wine for 15 minutes to concentrate its aromas and color.
In a large, wide pot, heat two more tablespoons olive oil and add the chopped vegetables. When they begin to brown, push the vegetables to the side and cover the bottom of the pot with the tail portions. Add enough beef broth just to cover and then add the reduced wine. Simmer for 5 to 6 hours, until the meat is very tender but does not fall from the bone. Add more beef broth as needed.
When tender, remove the tail pieces from the broth and place in an oven-proof casserole. Finely strain the broth. Return a half-cup to the tail pieces and keep warm. Place another half-cup in a clean pot and boil to reduce by two-thirds. Keep this malbec reduction warm. Just prior to serving, whisk a teaspoon of butter into the sauce until it is glossy and homogenous. Place a heaping tablespoon of potato-cauliflower purée in the center of each plate. Top each with a portion of oxtail, and then drizzle with a spoonful of the malbec sauce. Garnish with a sprig of rosemary and serve piping hot.