An hour or well ahead: Make the pickled onions by combining the water, apple vinegar, sugar, salt, and pepper in a small saucepan and bringing it to a boil over high heat. Lower the heat to medium-low and let simmer gently for 5 minutes. Add the onions, then remove from heat and let sit for an hour before refrigerating or using in the salad.About 30 minutes ahead: Place the cucumbers in a small bowl and sprinkle 1/4 teaspoon salt over them. Let sit for 30 minutes, refrigerated, to draw out the water.Drain the cucumbers and pat dry. Add the onions (drained of brine), cubed melon, apple vinegar, sesame oil, and perilla leaves and toss until well-combined. Add salt and pepper to taste, and serve immediately. Enjoy!