Preheat the oven to 425 degrees . In a small bowl, whisk the oil, lemon zest and juice, mustard and garlic; season the dressing with salt and pepper.
On a rimmed baking sheet, toss the potatoes, chicken, rosemary and sliced lemon with the dressing; season. Arrange the potato mixture and chicken, skin side up, in a single layer.
Roast, turning the potatoes once, until the potatoes are golden-brown and tender and the chicken is cooked through, 35 to 40 minutes.