Cover the sliced onions with a few splashes of the vinegar reduction. They should be covered but not much more. Toss to coat and leave them to soak for about 10 minutes until you are done with the tomatoes and basil.
Cut up the cherry tomatoes. Slice the basil into thin strips across the wide part of the leaf. Put tomatoes and basil into a serving bowl.
Drain the onions and add to the serving bowl. Season with salt and pepper. Mix gently and taste for salt. Serve immediately.
Leftovers are OK for a day or two. After that they start to break down.