Stand the cucumber pieces on their trimmed ends, and using a small knife, cut an X into each, stopping about halfway down. Season inside and out with 1-1/2 tablespoons of salt and let them sit in a shallow bowl at room temperature for 1-1/2 hours to soften.
Meanwhile, in a medium bowl, combine the onion, scallions, chives, saeujeot, garlic, ginger, gochugaru, and 2 tablespoons of water. Set aside.
Rinse the cucumbers well inside and out under cold running water and shake dry. Gently spread open the cucumbers and press about 1 heaping teaspoon of the chile mixture into each piece. Transfer them to a shallow glass or plastic container, standing them on their trimmed ends and packing them tightly together. Press any of the remaining chile mixture in and around the cucumbers and pour over any accumulated liquid. Cover and let sit at room temperature to ferment for at least 24 hours. Refrigerate until chilled, at least 30 minutes, and then serve.