Slice the onion as thinly as possible. Combine it with all the dressing ingredients and mix. (There's no need to soak the onion in water.)
After about 3 to 5 minutes, some moisture will come out of the onion slices and they will wilt.
The dressing is now done!
Cut the cucumber into 'jabara' (which means snake's belly). Make diagonal slices into it up to about halfway through the cucumber. Turn the cucumber over and make diagonal slices on the other side in the same way.
Once you put cuts into both sides, it will look like this.
Cut the tomato into bite sized pieces. Cut the jabara-cut cucumber into easy-to-eat pieces.
Pour the dressing over all and enjoy.
This dressing is perfect with Crepemetre-chan's "Hijiki Tomato Salad" See (Recipe ID: 1204844).
Story Behind this Recipe
This is a variation of a recipe a friend of mine taught me.
Slice the onion thinly so that it works well as part of the dressing. You can use tataki (bashed) cucumber instead of jabara cucumber. Sweet onions have lots of moisture, so be a bit generous with the salt.