pounds fingerling potatoes, halved crosswise if large
cup olive oil, divided
Kosher salt and freshly ground black pepper
cup Sherry vinegar or red wine vinegar
tablespoon whole grain mustard
jalapeño, thinly sliced into rounds, seeds removed if desired
cup (lightly packed) torn flat-leaf parsley leaves
Preheat oven to 450°. Toss potatoes with ¼ cup oil on a rimmed baking sheet; season with salt and pepper. Roast, tossing once, until golden brown and tender, 30–35 minutes. Let cool slightly, then lightly flatten. Meanwhile, whisk vinegar and mustard in a large bowl. Gradually whisk in remaining ¼ cup oil until emulsified; season with salt and pepper. Add potatoes, jalapeño, and parsley and toss; season with salt and pepper.