In a large skillet over medium-high heat, saute beef in the oil until cooked through, or until it reaches your desired level of doneness; remove from skillet and set aside.
Add mushrooms, onion and garlic to the skillet and simmer until mushrooms are reduced and onions are tender. Add flour and cook for about 1 minute, stirring constantly.
Add broth, mustard and thyme. Bring to a boil, reduce heat and simmer, uncovered, until sauce thickens. Add beef, sour cream and parsley; blend well.