Trim the spinach and wash thoroughly in cold water.
Cook the spinach in a little boiling salted water until just tender. Drain well, then refresh under cold running water. Drain again, squeezing out all the water from the spinach. Cut each leaf into 3 to 4 pieces and place in a bowl. Allow to cool.
Combine the sesame seed oil, sugar, vinegar, soy sauce and mustard in a bowl. Pour over the spinach and toss well before serving chilled.