Make the dressing: In a medium microwave-safe bowl, mix 3 tablespoons dashi, 2 tablespoons rice vinegar, 2 tablespoons mirin, 1.5 teaspoons soy sauce, and ¼ teaspoon kosher salt. Microwave for 1 minute, then give it a taste. If it feels too sour, add a little granulated sugar. Let it cool while you prep the other ingredients.
Prep the cucumber: Thinly slice ½ Japanese, Persian, English, or seedless cucumbers into coins, then sprinkle with ¼ teaspoon kosher salt and gently massage it with your hands. Let it sit for about 10 minutes until it releases water.
Prep the wakame: Soak ½ tablespoon cut dried wakame in a bowl of water for about 5 minutes. If using whole wakame, cut it into bite-sized pieces after soaking.
Assemble the salad: Squeeze the excess water out of the cucumbers and wakame with your hands, then toss them in a bowl with the dressing from step 1. Give it a good mix, and if you're adding any optional ingredients (see Note 3), now's the time!
Chill and serve: If the dressing is still warm, let it cool in the fridge until it’s chilled. Serve with the dressing and enjoy!