Pound the top round slices, and cut them into strips or bite-sized pieces.
Fry the onion in lard until translucent.
Add the meat, brown it on all sides, season with salt and pepper.
Add the sliced mushrooms, and fry together for a few minutes until the mushrooms release their liquid.
Pour in the beef stock, cover, and braise over low heat for 1-1.5 hours, until the meat is tender.
Whisk the sour cream together with the flour until smooth, stir in a few spoonfuls of the hot braising liquid, then pour the whole mixture back into the pot.
Heat for another 3-4 minutes, until the sauce thickens, stirring gently - don't let it boil hard.
Serve with boiled potatoes or dumplings.
Based on: Malatinszky Fanny szakácskönyve (a classic Hungarian cookbook), modernised. Calories and protein are estimated.