Remove all the fat and skin from the meat. Cut the meat into 1/2x2-inch strips. Sprinkle lightly with salt and pepper and let stand at room temperature for 1 hour.
Cook the onion and mushrooms in the butter until lightly browned. Sprinkle with the flour; mix thoroughly, and then stir in the soup stock, mustard, and sour cream. Bring to a boil. Season the sauce to taste with salt and pepper, and let it simmer for 10 minutes.
Melt the butter and quickly brown the meat strips in it along with the onion. Add the meat to the sauce. Cover and keep hot for 20 minutes to blend the flavors. Do not let it boil or simmer. When ready to serve, heat quickly over direct heat and serve immediately.