To make the soup
1 In a heavy-bottomed pan, sweat the sliced onions gently in the butter for 4-5 minutes. Add the sliced leeks and sweat for another 2 minutes. Add the potatoes, cover with boiling water and boil over a high heat until the potatoes are cooked.
2 Remove from the heat and blitz in a blender until smooth. Do this as quickly as possible to keep the lovely green colour.
To cook the ox tongue
3 Bring a large pot of seasoned water to the boil. Dice all the vegetables, add them to the pan with the garlic and bouquet garni and bring back to the boil. Then add the tongue and reduce the heat to a simmer. Cook for about 4 hours.
4 Once the tongue is tender, lift it out and carefully remove the skin while it is still warm. Also remove any glands, excess fat and gristle, then leave the tongue to rest and set in the fridge for at least 30 minutes. Cut it into cubes and then fry in a heated, oiled pan until golden brown and crispy on both sides.
To poach the eggs
5 Bring a pan of water to the boil and add the vinegar and a pinch of salt. Gently whisk the water to create a whirlpool effect, then drop in the eggs and cook for 3-4 minutes for soft yolks. Remove the eggs and place them on kitchen paper to drain.
To prepare the garnish
6 Sweat the leeks in a pan with a knob of butter and season with salt and pepper. Boil the potatoes for 4-5 minutes until soft.
7 Put some leek and potato garnish in the centre of each soup bowl and place an egg on top. Pour the soup around and serve with crispy ox tongue and cracked black pepper.