Thinly slice the mushroom. Heat the oil in a wok or frying pan, add the mushrooms and cook for a couple of minutes. Add the half of the sesame seeds, let them spatter, then then add the soy and mirin. Let the sauce bubble to reduce.
Meanwhile, finely slice the onions and radishes. Scatter the mixed leaves onto two plates.
The mushrooms should be cooked by now and slightly sticky. Add them to the salad leaves, then arrange the radish and onion pieces on top.
Drizzle with the sesame oil and vinegar, then scatter the remaining sesame seeds all over the salad.
Serve warm or at room temperature.