Put the potatoes, onions, garlic cloves, two tablespoons of the oil, salt and pepper into a 30cm-wide shallow casserole or ovenproof sauté pan. Toss everything around with your hands. Put the chicken on top, skin side up. Brush the remaining oil on the chicken and season.
Roast for 40 to 45 minutes, or until the chicken is golden and the potatoes tender when pierced with a sharp knife.
Squeeze the lemon juice over, then scatter on the zest, feta and dill.
Put the yogurt in a bowl and spoon the harissa on top. Serve the chicken with harissa yogurt on the side.