INGREDIENTS
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3 pounds fingerling potatoes, halved crosswise if large
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1/2 cup olive oil, divided
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Kosher salt and freshly ground black pepper
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1/4 cup Sherry vinegar or red wine vinegar
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1 tablespoon whole grain mustard
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1 jalapeño, thinly sliced into rounds, seeds removed if desired
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1/4 cup (lightly packed) torn flat-leaf parsley leaves