Place the dried wakame in a bowl and cover with cold water. Let soak 5–10 minutes until expanded and tender. Drain in a fine-mesh sieve and squeeze gently to remove excess water. Trim any thick stems and roughly chop into bite-sized pieces if needed.

Thinly slice the cucumber (preferably on a mandoline or at an angle) and lightly salt if you want to draw out moisture; after 5 minutes, blot with paper towel. Shred or julienne the carrot and thinly slice the red onion or scallion.

Combine the drained wakame, cucumber, carrot and onion in a mixing bowl. Toss gently to distribute ingredients evenly.

Sprinkle toasted sesame seeds over the salad just before serving.

Dressing

In a small bowl or jar, add rice vinegar, soy sauce, sesame oil, mirin, grated ginger, lemon juice (if using) and red pepper flakes (if using).

Whisk or shake vigorously until the mixture is emulsified and slightly thickened, about 20–30 seconds. Taste and adjust: add a little more soy for saltiness, more vinegar for acidity, or a pinch of sugar if you prefer sweeter.

Pour the dressing over the wakame and vegetables and toss gently to coat everything evenly. Let sit 2–5 minutes to allow flavors to meld before serving.