For pickled radish, combine sugar, vinegar, 250ml warm water and 2¼ tsp salt in a bowl and stir until sugar dissolves (1-2 minutes). Place radish in a plastic container, add vinegar mixture, cover and refrigerate until lightly pickled (1 hour).
Combine soy sauce, mirin and 500ml water in a saucepan and stir over medium heat until just simmering. Add mushrooms and simmer until mushrooms are tender (4-5 minutes), remove from heatand set aside.
Heat a large frying pan over high heat. Season steaks, add to pan and cook until caramelised (2-3 minutes each side for rare). Remove to a warm plate, cover loosely with foil and rest (5 minutes).
Spoon mushrooms and a little poaching broth over steaks, top with mizuna and pickled radish and serve immediately.