Set up a steamer or put a rack into a wok or deep-sided pan with a lid (a clear glass one is best so that you can keep an eye on the fish whilst it is cooking). Pour in enough water to come 5cm up the sides and bring to the boil.
Arrange the mackerel fillets on a heatproof plate lined with a large piece of parchment paper (you can use the excess to help you lift out the fish once it has cooked) and put on the rack. Scatter over the ginger, chilli and spring onions and drizzle over the sesame oil. Cover tightly and steam for 6-8 minutes, depending on the thickness of the fillets: they should have turned opaque and be flaking slightly but is still moist.
Meanwhile, make the salad. Place the edamame beans in a pan of boiling water with a pinch of salt and cook for 2-3 minutes or until tender. Drain and quickly refresh under cold running water, then toss with the radishes and mizuna or wild rocket. Combine the chilli flakes, lemon juice, soy and mirin in a bowl to make a chilli ponzu dressing.
Divide the edamame bean salad on plates and top each one with a piece of the aromatic mackerel. Drizzle over the chilli ponzu dressing and scatter over the coriander leaves to serve.