For the ox tongue, place the vegetables, parsley stalks, garlic and bouquet garni into a large saucepan. Cover with water and bring to the boil.
Add the ox tongue, reduce the heat and simmer for 4 hours, or until the meat is very tender.
Carefully remove the tongue from the pan. Remove the skin and any glands or excess fat and gristle. Set aside to rest in the fridge for at least 30 minutes, then, cut the tongue into cubes.
Heat a frying pan until hot, add the tongue cubes and fry until golden-brown and crisp on all sides.
Meanwhile, for the leek and potato soup, melt the butter in a saucepan over a gentle heat, until foaming. Fry the onions for 4-5 minutes, or until softened.
Add the leeks and fry for a further 2-3 minutes. Stir in the potatoes and cover with 1 litre/1pint 15¼fl oz water. Bring the mixture to the boil and cook for 10-15 minutes, or until the potatoes are tender.
Remove the pan from the heat and blend the mixture in a blender until smooth. Season, to taste, with salt and freshly ground black pepper.
To serve, heat the butter in a frying pan and fry the leeks for 4-5 minutes, or until softened. Season, to taste with salt and freshly ground black pepper.
Spoon some fried leeks and cooked potato into each of 4 serving bowls and top with a poached egg. Ladle the soup around the egg and top with the crisp ox tongue.