Slice the round steak into thin strips. Heat the oil in a skillet over medium-high heat. Add the beef and cook, stirring frequently, until well browned. Add the chopped onion, mushrooms, mustard, salt, and pepper. Cook, stirring frequently, until the beef is tender. Add the soup and water. Bring it to a simmer and let cook for 5 minutes. Reduce the heat to low and stir in the sour cream (do not let it boil or the sour cream will curdle). Stir to incorporate the sour cream and just let it heat through. Serve hot over cooked noodles.