I've eaten salads such as this at London's Nobu restaurant and it's essential to use really fresh fish. Ponzu is a kind of ready-made citrus dressing that can be bought from Japanese supermarkets as well as good delis and supermarkets.
150-200g very fresh white fish such as bass, bream, halibut etc
A small handful of small tasty salad leaves and herbs such as coriander, mizuna, celery, silver sorrel, rocket etc
1 medium carrot, peeled and shredded into matchstick-sized pieces
6-8 mangetout, trimmed and shredded
4-5cm piece of mooli, peeled and shredded into matchstick-sized pieces
4 spring onions, finely shredded on the angle
50-60g inoki mushrooms, trimmed
10g black fungus, soaked in cold water for a couple of hours
Drain the black fungus and shred as finely as possible; place in a bowl with the inoki mushrooms and Ponzu and leave for about 30 minutes.
Slice the fish with a sharp knife as thinly as possible and lay out on a tray. Toss the mushrooms with the salad leaves, shredded vegetables and spring onions, reserving a couple of spoonfuls of the liquid and season.
Spoon the reserved Ponzu over the fish. Arrange the salad on plates with the slices of fish and spoon over any excess Ponzu.