Preheat oven to 160°C (140°C fan-forced). Brush beef with half the oil. Season. Heat a large non-stick frying pan over high heat. Cook beef for 6-8 minutes or until browned all over. Transfer to a rack. Brush top of beef with mustard. Place onions, thyme, stock and wine in the base of a large roasting tin. Sit rack with beef over roasting tin. Cook beef for 50-55 minutes or until cooked to your liking. Set aside on a plate loosely covered with foil to rest for 15-20 minutes. Transfer onions and sauce to a saucepan. Simmer over medium-low heat for 15-20 minutes or until sauce reduces. Keep sauce warm.
Increase oven temperature to 200°C (180°C fan-forced). Spread sweet potato onto a large baking tray, drizzle with remaining oil, season and cook for 15-20 minutes or until golden and tender.
Slice beef. Reheat onions and sauce, if required. Serve beef with onions, sauce, sweet potatoes, snow peas and extra thyme.
Beef topside or rump roast would also work well in this recipe.
Resting your roast will ensure it is tender and will lock the juices in.
Leftover beef will be delicious in a crusty bread roll with salad for next day lunch.