Peel cucumbers, if desired. Cut in half lengthwise, and with a spoon, scoop out and discard seeds. Cut cucumbers diagonally into 1/4-inch-thick slices.
Put cucumbers in a colander set over a large bowl and mix with salt. Place a weight over cucumbers (a 1-gal. zip-lock plastic bag filled with water works well). Cover and chill at least 1 hour or up to 4 hours. Discard liquid.
In a bowl, whisk vinegar, lime juice, sugar, sesame oil, and chili flakes until well blended. Mix in drained cucumbers and the slivered onion. Sprinkle salad with the toasted sesame seeds.