Cook the noodles according to the packet instructions. Buckwheat noodles are delicate as they don’t contain gluten, so be careful not to overcook them (they will go gluggy and break up). Shake the noodles dry. Kelp noodles need to be soaked in water for only a few minutes, then drained.
For the dressing, combine the tamari, vinegar, mirin, sake, sesame seeds, spring onion and stock in a small saucepan and heat gently. Remove from the heat.
To serve, place the noodles in a large bowl or in individual bowls and arrange the mushrooms, bean sprouts, beans, carrot, spring onion and tofu on top. Transfer the dressing to a small jug or jugs to pour over the veggies when ready to eat. Serve the wasabi in a small dish at the table.