In a small bowl, mix all the ingredients (apart from the fish and green onions) together.
Smear a little of the dressing around a dinner plate. Arrange the fillets thickest sides out. Drizzle the remaining dressing over the top and spread it around.
Cover with cling film, punch a few holes in it then microwave on high for 2 minutes. Check and microwave a further 30 seconds at a time until the fish is just done. Allow to stand a couple of minutes then serve, pouring the juices over and garnishing with the sliced green onions.