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