Method

• 1.
  Preheat the oven to 200°C.
• 2.
  Place beef, fat-side down, in a 30cm x 40cm flameproof roasting pan over high heat and cook, turning, for 15 minutes or until evenly browned. Set aside.
• 3.
  Very thinly slice potatoes, carrots and parsnips into rounds using a mandoline (or the slicer attachment on a food processor) into a large bowl. Add onion, oil and thyme, and toss to coat.
• 4.
  Transfer vegetable mixture to cleaned roasting pan and stack slices in 2 rows along each of the longest sides of pan. Scatter with butter. Add garlic and half the rosemary to the middle of the pan and add stock and parmesan rinds, if using.

• 5.
  Top with beef and roast, basting with cooking juices every 30 minutes and covering with foil if meat becomes too dark, for 2 hours 15 minutes or until a meat thermometer inserted into thickest part of beef reads 60-65°C for medium-rare, or until cooked to your liking. Add remaining rosemary in last 30 minutes of beef cooking time to roast until crisp.
• 6.
  Remove beef from pan and set aside, loosely covered with foil, to rest. Using a ladle, carefully transfer pan juices to a saucepan over high heat, bring to the boil and cook for 10-15 minutes or until reduced by two-thirds.
• 7.
  When ready to serve, increase oven to 220°C. Return beef to pan with veg and cook for 10 minutes to crisp up and reheat. Finely grate over horseradish, if using, and serve with reduced cooking juices.