1. Preheat the oven to 400°F (200°C) mark 6.
2. Rinse the aubergines and place on the centre shelf of the oven to bake for about 30 minutes or until tender, turning them after 15 minutes. Take out of the oven and allow to cool. Slice off the ends, peel and cut lengthwise in quarters. Neatly remove any seed-bearing sections, if visible; cut the flesh into small, rectangular 'brick' shapes and heap these up on 4 small plates.
3. Mix 1 tbsp wasabi powder with just enough hot water to moisten and shape into 4 tiny balls; place one on each plate.
4. Peel a piece of Chinese yam or yamaimo and grate finely; you will need about 4 tbsp of its grated flesh; place a quarter of this neatly in a little heap beside the wasabi ball on each plate.
5. Heat a non-stick frying pan and toast the sesame seeds while stirring with a wooden spoon until they start to jump. Remove from the heat immediately and crush slightly with the back of the spoon. Sprinkle over the aubergine pieces.
6. Hand round tiny bowls of light soy sauce. Serve this dish in the Japanese way: mix some of the wasabi with a little soy sauce on one side of the plate, then stir in a little of the grated yamaimo. Take hold of a piece of aubergine with chopsticks or a fork and dip in the highly flavoured relish before eating.