1. Toss potatoes and onions in oil with herbs, a generous sprinkling of salt and a few grinds of pepper. Place potatoes, cut side down, in a single layer on a lipped cookie sheet. Set pan on lowest rack of cold oven; heat oven to 450 degrees. Roast until cut side is golden brown and potatoes are tender, 20 to 25 minutes.
2. Transfer potatoes to a large bowl; add capers and parsley.
3. Whisk vinegar, mayonnaise, garlic, a big pinch of salt and a couple of grinds of pepper in a 1-cup glass measuring cup. Slowly whisk in oil, first in droplets, then in a slow, steady stream. Pour dressing over warm salad; toss to coat. Serve warm.
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