Place the dried wakame in a medium bowl and pour 1 cup warm water over it. Let sit 5–10 minutes until fully rehydrated and expanded. Gently squeeze the wakame in a sieve or with your hands to remove excess water and chop any large pieces into 1–2 inch lengths.

Prepare cucumber and scallions

While the wakame rehydrates, thinly slice the cucumber (use a mandoline or knife) and slice the green parts of the scallions on a bias. If cucumber is watery, lightly salt and let sit 5 minutes, then blot with paper towel.

Assemble salad

Combine the drained, chopped wakame, sliced cucumber, and scallions in a mixing bowl. Toss gently to combine and distribute ingredients evenly.

Finish and garnish

Pour the dressing over the wakame mixture and toss to coat. Let sit 5–10 minutes for flavors to meld. Taste and adjust seasoning (add more soy for salt, more vinegar for brightness). Sprinkle toasted sesame seeds over the salad just before serving.

Dressing

Whisk together rice vinegar, soy sauce, sesame oil, mirin (if using), sugar or honey, grated ginger, red pepper flakes (if using) and salt in a small bowl until sugar dissolves and the dressing is emulsified. Taste and adjust balance—add a touch more vinegar for acidity or a pinch more sugar for sweetness.