Barley miso is a thick paste made from fermented barley and soybeans - it's used to flavour soups, stews and sauces and is available in jars from good supermarkets and oriental stores.
1.	To make the dressing, combine the apple and onion with the rice vinegar.
2.	Mix the sake and caster sugar in a small saucepan, and place over a moderate heat, whisking until the sugar has dissolved. Increase the heat momentarily to burn off any alcohol and leave to cool before stirring into the apple mixture.
3.	Using a mortar and pestle, grind the sesame seeds, garlic, ginger, mustard seeds, 7 spice powder and spring onion together, until they become a paste.
Add to the apple mixture.
4. Mix the dashi and miso together and add to the apple mixture.
5. Stir in the soy, sesame and rice oil, and set aside.
6. Season the tuna fillet with white pepper and sea salt, and rub with rice oil. Sear in a hot pan for 40 seconds on each side. Transfer the tuna into a heap of crushed ice - this stops the cooking process. Leave to cool before cutting into 4mm thick slices.
7. Combine the daikon and spring onion, and arrange on a plate. Top with the tuna and spoon over the dressing. Serve straight away with mixed salad leaves.