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Begin by mixing together the mustard, miso, vinegar and soy sauce in a bowl then gradually whisk in the two oils and lime juice. You could do this in a blender or food processor.
Put the sliced onion, cabbage and spinach into a bowl, pour over your dressing and mix to coat the leaves.
Check and remove any bones from the mackerel fillets then season with salt and pepper. Place the fillets on a tray and you can either cook in a combi-oven on grill mode or place under the grill. Remember presentation side down first and turn once after 5 minutes and crisp up the skin.
Serve the mackerel fillets on a cluster of the coleslaw and sprinkle with chopped spring onions. Drizzle a little of the dressing onto the plate also.
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