Thinly slice theonion and soak it in a bowl of cold water with ice cubes. Stir gently and letit sit for 30 minutes to reduce the sharpness of the onion.
While the onion is soaking, prepare the wakame. Place it in a bowl and pour in hot water to rehydrate. Soak for 1–2 minutes until fully expanded.
Drain the wakame and squeeze out any excess water. Then place it in a bowl of ice water to cool down quickly. Stir gently to help lower the temperature.
Once chilled, drain the wakame again thoroughly.
Now it's time to season. In a mixing bowl, combine the soaked onion, 1½ tbsp low-sodium soy sauce, 2 tbsp lemon juice, 1 tbsp sugar, 1 tsp sesame oil, and 1 tsp sesame seeds. Mix everything well until evenly coated.
Transfer the seaweedsalad to a glass container. Cover with plastic wrap, seal with a lid, give it agentle shake, and refrigerate for 1 hour to let the flavors develop.
After an hour, servethe desired portion and enjoy!