Preheat the oven to 180°C (160°C fan, gas 4). Cut the legs in half at the joint.
Peel and thinly slice the garlic. Tuck some slices under the skin of the legs.
Wash the potatoes and quarter lenghtways. Peel and slice the onions.
Place the legs on a baking tray, rub with 2 tbsp oil and season with salt and pepper. Bake in the preheated oven for 10 minutes.
Meanwhile, halve the lemon. Cut one half into slices and squeeze the juice of the other half. Mix the remaining oil with the mustard and lemon juice.
Pluck the thyme leaves and stir in. Season with salt and pepper.
Mix together the lemon slices, potatoes, onions and remaining garlic and add to the sauce.
Pour around the chicken pieces and cook for 40 minutes, turning occasionally.