1. In a heavy frying pan melt the 2 Tbsp butter and saute the onion until soft and just barely golden. Add the meat all at once and cook over high heat for just a few minutes, until cooked through. Season with salt and pepper. Set aside, but keep warm.
2. In a small saucepan melt the remaining 2 Tbsp butter. Mix together the flour and dry mustard and whisk into the butter. Cook for a minute, then gradually add the bouillion, stirring constantly, until a fairly thick sauce has been formed. Stir in the sour cream. Pour the sauce over the meat, check for seasoning, and heat through (careful not to boil).
