Rub the beef on all sides with plenty of salt and pepper. Scatter the onions and thyme sprigs in the bottom of a roasting tin and rest the beef on top. Drizzle with the oil.
Set the temperature to 225°C (437°F) and cook for 20 minutes, turning twice during cooking. Mix together the flour and mustard powder. Turn the meat so that the fat side is uppermost and sprinkle with the flour mixture. Cook for a further 15 minutes. (The beef will still be pink in the centre, so cook for a little longer if you prefer beef cooked through.) Transfer the meat to a board or platter, and leave to rest for 20 minutes before carving and while making the gravy.
Skim any fat from the juices in the tin, return to the halogen oven and fry the onions for 10 minutes. Pour in the wine and cook for a further 5 minutes. Season to taste with salt and pepper, and strain into a gravy jug. Serve hot with the beef and baby carrots or vegetables of your choice.