Make the sauce: In a small bowl, mix together the soy sauce, vinegar, gochujang, gochugaru, garlic, 1 tbsp sugar, sweetener, sesame seeds, and sesame oil. Place the sauce in the refrigerator for about 30 minutes.
While the sauce is chilling, make the salad: Prepare an ice bath. Wash your cucumbers and slice them into ⅛ inch thick pieces. Place them in a bowl and pour hot water into the bowl until the cucumbers are completely submerged. Allow them to soak in the hot water for 1 minute. Drain the cucumbers and place them in the ice bath for another minute, until the cucumbers are bright green. Drain and place back in the bowl, together with the sea salt and 1 tablespoon of sugar. Toss to coat and set aside for 10 minutes.
Drain the liquid that has collected in the bowl of cucumbers (no need to rinse). Gently pat them dry with a kitchen towel (no need to squeeze). Dry the bowl that was just emptied and place the cucumbers back in the bowl, together with the julienned onion, scallions, and sauce. Stir until the cucumbers are evenly coated. Serve immediately or store in the refrigerator in an airtight container for up to 10 days.
