1To make dressing, place soy sauce, lime juice, mirin, sugar sesame oil and 1 ½ tbs peanut oil in a jar and shake to combine.
2Preheat a lightly greased barbecue grill on medium-high heat. Sprinkle mushrooms with remaining oil, then barbecue for 4 minutes on each side, until golden. Set mushrooms aside to cool.
3Cook soba noodles in a large saucepan of rapidly boiling salted water until just tender, then rinse under cold water to refresh. Cut cooked mushrooms in half. Toss noodles with mushrooms, green onions, mint and dressing. Serve sprinkled with cashews.