Toss onion with vinegar in a small nonreactive bowl. Cover, and let stand at room temperature for 30 minutes.
Place onion with vinegar, tomatoes, and cucumber on a platter. Drizzle with oil, and top with basil. Season with salt and pepper, and serve.
This is a great salad. Great with tomatoes, cucumbers and basil from the garden. Will make again.
This is a super simple recipe and great with summer's heirlooms!