Cut steak crosswise into 1/4-inch strips.

Optionally, place steak in the freezer for about 30 minutes or until ice crystals begin to form to make slicing easier.

Thinly slice the onion halves.

Melt butter or margarine in a large skillet over medium heat.

Add the sliced onion and sauté until softened.

Add the beef strips to the skillet and brown on all sides.

Sprinkle flour over the beef and onions and stir to coat.

Gradually pour in the beef bouillon, stirring constantly to prevent lumps.

Stir in the prepared mustard and mushrooms.

Bring the sauce to a simmer and cook for 20-30 minutes, or until the beef is tender.

Reduce heat to low and gently stir in the sour cream. Do not boil.

Serve hot over cooked egg noodles.