Preheat oven to 120C. Put mustard seeds, salt, pepper and thyme in mortar and pound with pestle until lightly crushed. Add 60ml olive oil and pound until combined. Set aside.
Brush beef with remaining 1tbsp oil. Heat large non-stick frying pan over high heat and cook beef 1-2mins each side or until brown. Remove from pan and rub mustard mixture over beef. Put on rack over roasting dish and roast for 1hr 30mins for medium rare. Rest for 10mins before serving.