Japanese-style duck salad

This dish was served to me over twenty years ago in what was then Soho’s only Japanese restaurant, called Fuji (funny that). None of the ingredients, with the exception of soy, is particularly Japanese, or indeed at all difficult to obtain. English mustard may seem a rather strange inclusion, but the Japanese are very fond of it; their beloved wasabi (green horseradish paste served with raw fish) is in fact nothing more than English-style mustard powder with green colouring and horseradish