1. Place the ox tongue in a large pan and cover with cold water. To remove any impurities, bring the water to the boil and then remove from the heat and discard the water.
2. Cover the tongue with fresh water, return to the heat and bring to the boil, then add the mirepoix of vegetables (carrots, onion and leek) with the garlic, thyme and bay leaf. Leave simmering and cook for 3-3 ½ hours. As with any braised meat a good way to check that it is cooked is to take a roasting fork and place it into the meat. If you hold the fork up, the meat should easily fall off and back into the stock.
3. Once cooked, peel off and discard the coarse skin of the ox tongue. Set the stock aside and reserve. Allow the ox tongue to cool and chill before cutting into cubes.
4. Meanwhile, peel the potatoes and with a serrated knife carve each into a cylinder shape resembling bone marrow. Then with a baller, scoop out the inside to create a well in each potato.
5. Heat a pan of duck fat and carefully place the carved potatoes in, simmering until cooked (about 10-15 minutes). Remove the potatoes and leave to drain. Then place them in a dry frying pan and fry until golden in colour.
6. Bring another pan of water to the boil, place the bone marrow into the pan with one clove of chopped garlic, a sprig of thyme and salt to season, poaching until soft (about 10 minutes) or cook according to the pack instructions scattered with one clove of garlic and thyme. Remove the cooked bone marrow from the pan and set aside.
7. Meanwhile sauté the ox tongue in a frying pan for 3-4 minutes until crispy. Add the mushrooms, cooking for a further 1-2 minutes. Add one clove of chopped garlic and a teaspoon of chopped parsley.
8. Chop the bone marrow and season with chopped shallots, chopped parsley and a dash of sherry vinegar. Place a teaspoon of bone marrow and a piece of ox tongue inside each potato and serve with the remaining ox tongue and mushroom mixture around the outside.