Wipe the oxtails dry with a paper towel. Put seasoned flour in a resealable plastic bag, then add the Oxtail and shake to coat with flour.
Heat the butter and oil in the potjie pot and sauté bacon pieces. Remove bacon, add the oxtail to the pot and brown in the bacon fat. When each piece is browned all over, remove and drain on paper towels while keeping warm.
Add the 4 finely diced carrots to the pot together with the chopped onions and the leeks, and sautée until softened. Return the oxtail and bacon to the pot and add the bay leaf, peppercorns, garlic, tomato paste, beef stock and red wine. Bring slowly to the boil. Cover tightly with a lid and allow to cook undisturbed for 4 hours.
One hour before serving, add the remaining carrots and mushrooms and continue cooking slowly, without stirring. If you want a thicker sauce, stir in some cornstarch mixed with cold water just before serving.
