Slice the cucumber thinly and cut the onion perpendicular to its fibers. Soak the onion slices in water to reduce their sharpness.

Rehydrate the dried seaweed in water according to package instructions, then drain.

Combine the cucumber, onion, and rehydrated seaweed in a bowl.

In a separate bowl, mix sugar, salt, vinegar, and grated ginger to create the dressing.