Slice Japanese Cucumber (0.5 pound) crosswise 1/8-inch thick and toss with Fine Sea Salt (1 1/4 teaspoon). Let stand 30 minutes, then rinse well and squeeze out excess liquid with your hands.
Halve the Fuji Apple (1) half lengthwise and cut out core. With a mandoline or sharp knife, slice crosswise 1/8-inch thick. Set the other half aside for later.
Toss the apple with cucumbers and Water (2 cup), Rice Vinegar (1/3 cup), Fresh Ginger (1 tablespoon), Granulated Sugar (1/4 cup), and Korean Chili Flakes (1 pinch). Marinate, chilled, turning occasionally, for 1 day, or just eat immediately. No need to marinate these cucumbers and apple slices for very long.
With the other half of the granny smith apple, peel and grate 2 tablespoons of it.
Whisk together Mirin (1/4 cup), grated Apple, Soy Sauce (2 tablespoon), Light Corn Syrup (1 tablespoon), Scallion (1 1/2 tablespoon), Gochujang (1 tablespoon), Fresh Ginger (1 tablespoon), Garlic (2 clove), Sesame Oil (1 1/2 teaspoon), and Rice Vinegar (1 1/2 teaspoon).
Add Skirt Steak (1). Cover and chill overnight. If you can’t marinate overnight, try for at least an hour.
Meanwhile, prepare barbecue (high heat) or place a cast iron skillet or a stainless steel skillet over high heat. Remove sliced steak from marinade, place on a plate or in a colander, and pat dry with paper towels.
Cook steaks for about 1 minute or less per side. Transfer to a platter. Let stand 5 minutes, then serve with the cucumbers.