Heat 1/2 the oil in a large saucepan over medium-high heat. Add the beef loin to the pan and cook for 4 minutes or until just browned. Transfer to a heatproof bowl and set aside.
Heat remaining oil in same pan over medium-high heat. Add onion and mushrooms. Cook for 3 minutes or until onion starts to brown, stirring continuously. Add garlic and paprika. Stir for 30 seconds or until fragrant. Add Worcestershire sauce and tomato paste. Stir to coat. Add beef stock and 2 cups of water. Bring to the boil. Add spaghetti. Cook for 8 minutes or until pasta is tender, stirring occasionally.
Return beef loin to pan with zucchini. Stir. Cook for 2 minutes or until heated through. Stir in 1/2 the sour cream. Serve stroganoff topped with remaining sour cream and sprinkled with parsley and extra paprika.