Preheat oven to 450 degrees. On a nonstick rimmed baking sheet, toss potatoes with 3 tablespoons olive oil; season generously with coarse salt and ground pepper. Turn cut sides down. Bake, turning once, until golden brown and fork-tender, 30 to 45 minutes.
In a large bowl, whisk together white-wine vinegar, grainy mustard, and 2 tablespoons olive oil. Add hot potatoes; season with salt and pepper. Toss to coat evenly. Serve warm or at room temperature.