Partially peel cucumbers lengthwise, leaving thin green stripes around the cucumber. Halve cucumbers lengthwise. Scoop seeds from cucumber halves with a spoon. Discard seeds. Slice cucumbers thinly and put in a bowl.
Add salt to cucumber and mix with hand until cucumber is limp; set aside for 45 minutes.
Transfer cucumbers to a strainer and squeeze as much liquid out as possible.
Mix lemon juice, sugar, and vinegar together in a large bowl until the sugar dissolves.
Add the drained cucumber and toss to coat. Spoon into small bowls and top each portion with toasted sesame seeds.
