Cut the roots off the komatsuna and slice into 4cm pieces. Place in a microwave-safe container, add 1 tbsp water, cover loosely with plastic wrap, and microwave at 600W for 3 minutes. Cool in cold water, drain, and squeeze out excess water.

Cut off both ends of the cucumber, slice it in half lengthwise, and cut into thin diagonal slices. Place in a bowl, sprinkle with a pinch of salt, and massage lightly. Let sit for 5 minutes, then rinse and squeeze out excess water.