Place diced vegetables, parsley stalks, garlic, and bouquet garni into a large saucepan and cover with water.
Bring to a boil, add the ox tongue, and simmer for 4 hours until the meat is tender.
Remove the tongue, discard skin and fat, chill for 30 minutes, then cut into cubes.
Fry the tongue cubes in a hot pan until golden-brown and crisp.
Melt butter in a saucepan and fry sliced onions for 5 minutes until softened.
Add leeks and fry for 3 minutes, then stir in potatoes and 1 liter of water.
Boil for 15 minutes until potatoes are tender, then blend until smooth and season.
Fry the remaining leeks in butter until soft and season with salt and pepper.
Serve in bowls by layering fried leeks, boiled potato, a poached egg, soup, and crispy tongue.