Put a bundles of soba in boiling water for about 6 minutes over high heat. Rinse under running water and drain. Cut nori into 1-inch-long shreds with kitchen scissors. For dipping sauce, mix Hontsuyu and water, and half fill small individual bowls with the sauce. Refresh noodles under cold running water a second time and arrange them on individual bamboo mats placed on large plates, or directlly onto plates. Sprinkle the nori on top. Serve with small plates of the chopped green onions and wasabi paste and the dipping sauce.