Thinly slice the yellow onion into rounds and then quarter the rounds.
Put vegetables and cherry tomatoes in a bowl for serving.
Add the vinegar and salt and pepper, and use your hands or salad tongs to thoroughly coat the vegetables (make sure you don’t add too much balsamic vinegar. It doesn’t look good to have brown cucumbers!).
Serve the salad right away, or cover with plastic wrap and let it sit a couple hours before serving (the longer it sits in the vinegar, the more tangy the cucumbers will become)