1. Mix together the mirin, soy sauce, brown sugar, and sesame oil to make a marinade. Place the marinade in a resealable bag with the sliced steak and leave to marinate for an hour or overnight.
2. Whisk together the dressing ingredients.
3. Heat the rapeseed oil in a large pan or wok until hot, then carefully lift the steak slices out of the marinade, add to the pan and stir-fry quickly for a minute. Remove from the pan and leave to rest for 5 minutes.
4. Dress the watercress, asparagus, and edamame beans with the dressing. Arrange on a serving plate with the steak and top with the spring onions and sesame seeds.
TIP: The asparagus edamame salad would also be a great accompaniment to plain grilled steak, lamb or chicken.