Thinly slice 1 red onion and add it to a bowl with 2 tablespoons lemon juice, ⅛ teaspoon salt, and ¼ teaspoon red pepper flakes.Massage them with your fingertips to distribute the condiments, then set them aside for 15 minutes while you prepare the salad.
Wash and dry 1½ pounds cucumbers, remove the tips as they can be bitter, then put them flat on a cutting board and smack them with a wooden spoon until they crack.
Cut the smacked cucumbers in half lengthwise and scoop out the seeds, if any, with a teaspoon.
Slice cucumbers into small bite-size chunks.
Transfer the cucumber into a colander and toss it with2 teaspoons salt. Now let drain for 15 minutes.
To a small bowl, add 2 tablespoons rice vinegar, 1 tablespoon soy sauce, 1 tablespoons maple syrup, ½ tablespoon sesame oil, 1 teaspoon sriracha sauce, 1 inch ginger (grated), and 1 clove garlic (grated). Whisk and set aside.
Shake the water off the cucumbers (do not rinse them with water), then transfer them into a bowl and toss them with the dressing.
Garnish with 1 tablespoon sesame seeds and pickled red onions.