Put the tuna steaks in a sealable bag, add all the marinade ingredients, except for the oil, and gently massage them into the tuna. Seal the bag, making sure there is no air trapped inside and place in the fridge to marinate for 6–8 hours.
To make the salad, bring a large pan of salted water to the boil and blanch the beansprouts for 2 minutes then refresh in cold water, drain and set aside.
Mix the remaining salad ingredients together in a bowl, add the beansprouts and set aside.
To make the dressing, mix the sugar syrup, dashi, rice wine vinegar, fish sauce, a tablespoon of lemon juice and the soy sauce together in a bowl.
Remove the tuna from the fridge at least 30 minutes before cooking.
Heat a griddle pan over a high heat and rub with a little vegetable oil. Remove the tuna from the bag and pat dry, then cook the tuna for 2 minutes on each side. Remove from the pan and thinly slice.
Blanch the pak choi in boiling water for 2 minutes and refresh under cold water. Drain and place on the griddle pan next to the tuna for 2 minutes each side.
Divide the salad between the plates and top each one with some of the tuna and the pak choi. Spoon the dressing over and serve.