Fry the onion in the lard until translucent, sprinkle with the flour, and fry further until light brown.
Pour in the water, stir until smooth, add the bay leaf, salt it, and bring to a boil.
Pour in the vinegar to taste, and keep boiling.
Crack the eggs into the soup one at a time per person, while it's boiling, keeping the yolks whole; cook for 3-4 minutes until the white sets but the yolk is still a little runny.
Mix the sour cream into the soup.
Serve carefully, with one poached egg per bowl.
Based on: Malatinszky Fanny szakácskönyve (a classic Hungarian cookbook), modernised. Calories and protein are estimated.