Wash the vegetables and clean them, dice them. Rip the cauliflower into florets. Put the potatoes, onions, carrot, cauliflower and celery leaves in a pot with salted water and boil until they soften.
Blend with the butter but not into a puree, leave some larger pieces of cauliflower. Season to taste. In a bowl, beat the egg yolk with the milk and pour it into the pot in a thin trickle.
Stir well. Serve with the croutons from the brown bread and freshly ground black pepper.