Method

• 1.
  Mix 2 tablespoons of the oil in a dish with the garlic and thyme. Add the beef fillet and coat in the mixture. Cover and refrigerate for 2-3 hours.
• 2.
  Place remaining oil, vinegar and mustard in a bowl and whisk to combine. Season with salt and pepper.
• 3.
  Preheat the oven to 190°C. Heat butter in a large frypan over high heat, add the mushrooms and cook 2-3 minutes until just wilted. Add stock and heat through, then add mixture to bowl with vinaigrette. Add half the chives and set aside.
• 4.
  Return pan to heat and brown beef all over, then place beef in a roasting pan and roast for 20 minutes for rare, or until cooked to your liking. Remove and set aside to rest for 10 minutes.
• 5.
  To serve, place a thick slice of beef on each plate. Add any roasting pan juices to the mushroom vinaigrette, then spoon over the beef and garnish with chives. Serve with watercress.