Pound the sirloin steaks, season with salt, and coat in flour.
Fry in hot lard over high heat, 2-3 minutes per side, until both sides are browned, then take out and keep warm.
Add the finely chopped onion to the same pan, and fry in the remaining fat over medium heat until translucent.
Pour in the water or beef stock, and simmer for a few minutes to dissolve the browned bits stuck to the bottom of the pan.
Stir in the sour cream, season with salt and pepper, and heat gently for 2-3 minutes until the sauce thickens slightly (don't let it boil hard, or the sour cream may curdle).
Return the steaks to the sauce, or pour the sauce over the steaks, and serve immediately.
Serve with mashed potatoes or steamed rice.
Based on: Malatinszky Fanny szakácskönyve (a classic Hungarian cookbook), modernised. Calories and protein are estimated.