Place the beef in a shallow dish, drizzle with the hoisin sauce and half of the oil, set aside to marinate for 30 minutes.
Mix the sauce ingredients together in a small bowl.
Heat a frying pan over a medium-high heat and cook the steak for 3-4 minutes on each side for medium - rare. Take care not to let the sauce burn.
While the meat is resting, wipe out the pan, return to the heat with a little more oil and cook the onion, capsicum, corn cobs and bok choy until just tender.
Pour the sauce over the vegetables and toss to combine. Season to taste with a little salt.
Slice the steak and serve with the vegetables, sprinkle with sesame seeds.