Rinse the burdock and lightly scrape off the skin with the back of a knife. Do not peel it fully - the flavor is just under the skin.
Cut the burdock into 5 cm (2 inch) lengths, then into thin matchsticks. Soak in water for about 5 minutes, then drain.
Cut the carrot into matchsticks the same size. Cut the cucumber the same way, sprinkle with a pinch of salt, and set aside.
Bring a pot of water to a boil. Add the burdock and boil for about 1 minute after the water returns to a boil, until it loses its raw edge but still snaps. Scoop out and drain.
Boil the carrot in the same water for about 1 minute (or boil it together with the burdock). Drain well and let the vegetables cool for a few minutes.
Stir the mayonnaise and soy sauce together in a large bowl. Squeeze the cucumber gently, then add it with the burdock and carrot and toss until coated. Taste and add a few drops of soy sauce if needed.
