Ingredients

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

Instructions

1. Preheat oven to 450°. Toss potatoes with 1/4 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.
2. Meanwhile, whisk vinegar and mustard in a large bowl. Gradually whisk in remaining 1/4 cup oil until emulsified; season with salt and pepper. Add potatoes, jalapeño, and parsley and toss; season with salt and pepper.