Slice meat along the grain into strips 1 inch long by ¼ inch wide; season with salt and pepper.
Toss in 4 tablespoons flour, reserving 1 tablespoon for sauce.
In a large saucepan, over medium high heat, cook meat until browned; transfer to a plate; set aside.
Add onions and mushrooms to pan; continue cooking until light brown; transfer these to plate with meat.
Add butter and remaining 1 tablespoon flour to pan, stirring constantly over low heat for 2 minutes.
Stir in demi-glace, beef broth and sour cream; add salt and pepper to taste.
Add meat, onions and mushrooms; heat very gently on low 15 minutes; do not simmer.