Melt butter in a large skillet over medium heat.

Add mushrooms and sliced onion to the skillet and cook for 3 minutes, until softened.

Remove mushrooms and onion from the skillet and set aside.

Cut the beef into 1 1/2-inch strips or chunks.

Dredge the beef in flour, salt, and pepper.

Add more butter or shortening to the skillet, if needed.

Brown the beef quickly on all sides.

Add stock or bouillon cubes and water to the skillet.

Stir in the cooked mushrooms, onion, and brown gravy flavoring.

Cover the skillet tightly and simmer for 1 hour, or until the meat is tender.

Ten minutes before serving, stir in sour cream.

Serve hot over noodles or mashed potatoes.