Preheat oven to 120°C (250°F). Place the mustard seeds, salt, pepper and thyme in a mortar and pestle and pound until lightly crushed. Add the oil and pound until combined. Set aside.
Brush the beef with the extra oil. Heat a large non-stick frying pan over high heat and cook the beef for 1–2 minutes each side or until brown. Remove from the pan.
Secure with kitchen string and rub the mustard mixture over the beef. Place on a rack in a baking dish and roast for 1½ hours for medium rare. Serves 4–6.
+ After roasting, cover the meat with foil and rest in a warm place for at least 10 minutes to allow the meat to relax.