I used a mandolin to evenly and thinly slice all of the cucumbers. Toss the cucumbers in a large bowl with the sugar and salt. Let them rest for 30 minutes before transferring them to a colander for a good rinse. Drain them and then press them with a paper towel to absorb the excess moisture. Return them to your large bowl.In a small bowl, stir together the vinegar, garlic, chile flakes and sesame seeds. Pour this over your cucumbers and toss to coat.Cover and chill for at least 2 hours. When ready to serve, pour the cucumbers onto your serving dish and sprinkle with the spring onions. Garnish with a pinch more of sesame seed and red pepper flakes.