beef sirloin, boneless cut into strips
1Spread beef strips on a baking sheet, sprinkle with salt and pepper, top with onion slices and let stand at room temperature for 2 hours.
2Melt 2 tablespoons butter in a skillet and stir in flour.
3Mix in stock and heat, stirring until thickened.
4Blend in mustard and remove from heat.
5In another skillet, melt remaining butter, add beef and onion and saute until browned.
6Add the beef (but not the onion) to the sauce and simmer 15 minutes.
7Remove from heat and stir in sour cream (at room temperature).
8Serve onions separately or reserve for another use.