First make the dressing. Mix onion, soy sauce, rice wine vinegar, sugar, mustard powder and water. Just mix well with pepper and mix until the sugar is dissolved.
Then add the grape seed oil and the sesame oil little by little and mix until it comes together. Heat a tablespoon of grape seed oil in a frying pan over medium heat.
Sprinkle salt and pepper on both sides of the tuna and press well. Drizzle with a little dressing on both sides.
Fry the tuna briefly on both sides, about 40 seconds a side.
Place the tuna on a board and cut thin slices with a razor sharp knife.
Prepare the plates. Place the cucumber with the radishes on top in the centre of the plate. Roll the thinly sliced tuna and put it around it. Drizzle generously with the dressing and top it all off with some micro salad. Enjoy your meal!