First make the salad dressing: make a paste with the wasabi powder and the water. Add to a bowl with the remaining dressing ingredients and whisk the mixture until blended.
Cook the salmon: brush a flat tray with oil and cut the fish into 5cm chunks. Combine the kecap manis and the hoisin and brush the fish with it. Lay the fish on the tray and cook under the grill for 3 minutes: it should still be reddish in the centre. Remove from the heat.
Mix the salad ingredients together in a bowl and toss with the dressing.
Flake the fish with your fingers and mix through the salad. Arrange on plates and serve immediately.