Preheat the oven to 220ºC, gas mark 7. Put the potatoes, fennel and onion into a large roasting tray, toss with the oil and season well. Tuck in the thyme sprigs and roast for 10 minutes. Sit the chicken legs on top, skin-side up. Season, then roast for 25 minutes more.
Remove the tray from the oven and pour over the stock, taking care to avoid the chicken skin. Reduce the oven temperature to 200ºC, gas mark 6, then bake for 15-20 minutes more, until the vegetables are tender and the chicken is cooked through with no pink meat and the juices run clear.
Toss the apple in the mustard. Plate up the vegetables and chicken, scatter with a few thyme leaves and add a spoonful of mustardtossed apple on the side.
