Grate the ginger, then mix with the mirin, sugar and soy sauce.
Dry-fry the sesame seeds in a frying pan until golden brown and then crush with a pestle & mortar until you get a paste. Add to the dressing and mix well.
Cut the woody end off the bean, rinse them then blanch in boiling water for 2 minutes, or steam above simmering water for 3-4 minutes, until tender but still al dente. It is important for the beans to remain crispy, so don't overcook them! Set aside (covered).
Lay the beans on a platter, spoon over the sesame paste and scatter some sesame seeds on top. Serve warm or at room temperature.