Preheat oven to 200°C. Place beef into a large roasting pan. Combine thyme, garlic, 2 tsp of the oil, salt and pepper in a bowl. Rub mixture over beef.
Make cuts into each potato at 5mm intervals, 2/3 of the way through. Place potatoes, carrot and parsnip into a bowl. Add remaining oil and toss well to coat. Arrange vegetables in baking dish around beef. Roast for 30 minutes. Turn carrot and parsnip. Roast for a further 40 minutes until vegetables are golden and crisp. If the beef is colouring too much, cover it loosely with foil.
Transfer beef to a board and vegetables to a warm plate. Cover both with foil.
Drain cooking juices from roasting pan, leaving 1 tbs. Place pan over medium heat. Add half of the butter. When melted, sprinkle flour over and whisk to combine. Cook for 1 minute. Slowly add stock, whisking constantly until smooth. Stir in sauce and bring to the boil. Simmer for 3 minutes or until slightly thickened. Strain and transfer to a serving jug.
Slice beef. Add remaining butter to vegetables. Serve beef and vegetables with gravy.