1. Preheat the oven to 200ºC, gas mark 6. Arrange the onion slices in a small-to-medium roasting tin. Place the chicken on top, rub with 1 tbsp of the oil and season well, then roast for 20 minutes. Pour the ale into the tin around the chicken and roast for a further 40 minutes.
2. Make a glaze by mixing together the barbecue sauce, maple syrup, garlic and thyme leaves in a bowl. In a separate roasting tin, toss the sweet potatoes with the remaining 2 tbsp oil, cumin seeds and some seasoning. Remove the chicken from the oven, and put the sweet potatoes in the oven to roast. Brush the maple glaze all over the chicken. Return to the oven with the sweet potatoes and roast for a further 15-20 minutes, until the juices run clear when a skewer is inserted into the thickest part of the thigh, and no pink meat remains.
3. When the chicken is cooked, put on a carving board to rest, covered with foil. Turn the sweet potatoes in the oil and roast for a further 15-20 minutes, until crispy and tender.
4. Meanwhile, for the beer gravy, strain the cooking juices through a sieve into a jug. Reserving 2 tbsp, spoon off any fat. Top up the juices with the stock to make 600ml. Put the reserved fat into a pan and, over a medium heat, stir in the flour and some seasoning. Cook for 1 minute, then gradually stir in the stock. Bring to the boil then simmer for 5-10 minutes to reduce.
5. In the meantime, to make the slaw, mix all the winter slaw ingredients in a serving bowl and season to taste.
6. Serve the chicken with the roast sweet potatoes, winter slaw and a warmed jug full of the beer gravy.