Heat the oven to 400°F (200°C).
Cut the peeled russet potatoes into chunks and slice the leeks into rounds, then spread them evenly in a large oven-safe skillet or braiser. Drizzle with 2 tablespoons of olive oil and season lightly with salt and pepper, tossing to coat.
Arrange the bone-in chicken pieces on top of the potato and leek mixture.
Combine all the lemon herb sauce ingredients in a bowl, whisking until smooth and well mixed.
Spoon the sauce liberally over each chicken piece and drizzle any leftover sauce over the vegetables.
Cover the pan tightly and bake for 40 to 45 minutes.
Increase the oven temperature to 425°F (220°C), remove the cover and continue to bake uncovered for another 30 to 45 minutes, until the chicken skin crisps and the potatoes are tender.
Optionally, place the pan under the broiler for 2 to 3 minutes at the end to brown and crisp the top.
Before serving, spoon some of the pan juices over the chicken and sprinkle with additional chopped parsley for a fresh finish.