Braise it in the lard in a pot, covered, over low heat, stirring often so it doesn't burn, until it cooks down completely soft and mushy (about 10 minutes).
Sprinkle with the flour, mix in, then pour in the water or broth.
Bring to a boil, salt it, and cook over low heat for another 5-8 minutes.
When serving, mix in the egg, the butter and the sour cream.
Serve with toasted bread cubes.
Based on: Malatinszky Fanny szakácskönyve (a classic Hungarian cookbook), modernised. Calories and protein are estimated.