Dry the meat well. Rub it all over with the oil and drizzle over the lemon juice. Place half the onion slices and 1 thyme sprig in a dish, put the beef on top and cover with the remaining onions and thyme sprig. Season with salt and pepper. Cover and leave the beef in a cool place to marinate for at least 3 hours.
Discard the onions. Place the beef on a rack over a roasting tin to catch the meat juices. Put into the top of a preheated oven, 220°C (425°F), Gas Mark 7. For rare beef, allow 15 minutes per 500 g (1 lb) plus 15 minutes. For medium-done beef, allow 20 minutes per 500 g (1 lb) plus 20 minutes. After 1 hour of cooking, reduce the temperature to 190°C (375°F), Gas Mark 5. Baste occasionally with the pan juices.
Remove the beef from the roasting tin and allow it to rest on the rack, covered with a tent of foil. Carve the beef into slices and serve with Red Wine Sauce (see below) and roast potatoes and vegetables, if liked.