01 - Preheat the oven to 425°F.
02 - Toss the potatoes with 2 tablespoons olive oil, salt, and pepper on a baking tray. Roast for 30–35 minutes, turning once, until golden and crisp on the outside and tender inside.
03 - Let the potatoes cool for 10 minutes, then transfer to a large bowl.
04 - While potatoes roast, prepare the dressing: whisk together the remaining olive oil, Dijon mustard, white wine vinegar, lemon juice, garlic, and a pinch of salt and pepper in a small bowl.
05 - Add the red onion, celery, radishes, chives, and parsley to the warm potatoes. Drizzle with the dressing and gently toss to coat.
06 - Taste and adjust seasoning if necessary. Serve warm or at room temperature.