Place onion in a small bowl. Cover in cold water. Squeeze and release the onion slices about 5–6 times. Drain water and repeat squeeze-and-rinse cycle 2 more times. Place the slices back in bowl. Cover with cold water while assembling the salad.
In a small bowl, whisk together sherry vinegar, olive oil, honey, 2 pinches kosher salt and several grinds fresh ground pepper.
Drain the onions and squeeze them dry. In a large bowl, gently mix together onions with tomatoes, peaches, beans, basil and vinegar dressing, and a few pinches kosher salt. Sprinkle with crumbled feta cheese. Serve.