1. Warm oil in a large skillet over high heat. Sprinkle beef with salt and pepper; add to skillet with onion. Cook, stirring, for 2 minutes. Stir in mushrooms and garlic; sauté for 2 minutes. Reduce heat to medium, stir in broth, cover and cook 3 minutes.
2. In a bowl, whisk sour cream and flour. Whisk in some broth from skillet to loosen mixture. Add sour cream mixture to skillet. Cook, stirring, until thickened, about 3 minutes. Stir in mustard and parsley. Season with salt and pepper and serve.