Bring a large saucepan of water to the boil over high heat. Add the noodles and stir to separate. Return to the boil, then add 250ml cold water. Repeat this step three times as the water just comes to the boil. Test a piece of soba - it should be tender to the bite, cooked through but not mushy. If it's not quite done, repeat one more time. Drain and rinse under cold water until the noodles are cold. Cut into 10cm lengths using scissors. Toss with 2 teaspoons of the sesame oil.
Place the chicken in a non- metallic bowl. Combine the ginger, soy, mirin, sake and the remaining sesame oil, then pour over the chicken and turn until well coated. Cover and refrigerate for 2 hours.
Line a bamboo steamer with baking paper. Remove the chicken, reserving the marinade and place the fillets in the steamer. Place the streamer over a wok or saucepan of simmering water, making sure the base doesn't touch the water. Cover and stream for 8-10 minutes or until cooked through. Remove and allow to cool completely.
Place the reserved marinade in a small saucepan with the dashi granules, sugar, rice vinegar and 90ml water. Bring to the boil, then reduce the heat and simmer for 4-5 minutes. Remove and cool completely in the fridge.
Place the cucumber, pickled ginger, onion and noodles in a large bowl. Cut the chicken into 5 mm strips on the diagonal and add to the noodles. Pour on the dressing and toss well. Serve sprinkled with the sesame seeds.