Cut the beef into 2 inch strips. In a large frying pan simmer the onions in butter until they are soft. Add button mushrooms, and stir, over moderate heat, for 3 to 4 minutes longer, until their juices have evaporated, adding more butter if necessary. Remove the onions and mushrooms with a slotted spoon. Melt another 2 tbsp of butter in the pan and brown the beef strips over a high heat, stirring constantly, for 4 to 5 minutes. The strips should not be well done. Return the onions and mushrooms to the pan. Season to taste with salt and pepper, and cook for 1 minute longer. Remove from heat. Add sour cream and mild mustard. Stir over low heat until hot without boiling. Serve immediately.{$7e}(Original recipe for 4)
