Peel and finely dice the onions and garlic. Clean, wash and cut the leek into smaller pieces, except for a longer piece for garnishing. Peel potatoes and cut into cubes (put some cubes aside).
Heat the fat in a saucepan and sauté the onions and garlic. Add leek and potatoes and fry briefly. Deglaze with stock and milk, bring to the boil, cover and cook over medium heat for about 15 minutes.
In the meantime, heat the oil in a pan and fry the potato pieces for about 8 minutes until golden brown. Cut the rest of the leek into strips and blanch in boiling salted water for about 1 minute. Remove.
Take the soup off the stove and puree it finely. Season to taste with salt, pepper and sugar. Whip the cream lightly and stir into the soup, except for 2 tablespoons for serving. Warm the soup again briefly. Serve with a dollop of cream, potato pieces and leek strips in deep plates.
Sprinkle with white pepper.