Place the carcass and any bones in a large pan and use a rolling pin to smash them all up. Cover with 1 litre of water, bring to the boil, then simmer for at least 30 minutes, skimming away any scum from the surface.
Meanwhile, finely slice the bacon and place in a large casserole pan on a medium heat with a lug of oil, while you peel and chop the onions, carrots, and potatoes or celeriac into 2cm dice.
Add to the pan along with the thyme sprigs and bay leaves. Cook for 10 minutes, stirring regularly.
Halve and stir in the mushrooms, along with the leftover chicken and plain flour.
Pour the stock through a sieve straight into the pan (topping up with a little water, if needed) and let it simmer for 20 minutes, or until thick and delicious, while you start your dumplings.
Place the self-raising flour and a pinch of sea salt in a mixing bowl. Coarsely grate in the butter and rub together until it resembles breadcrumbs.
Add 100ml of cold water and bring into a ball of dough. Divide into 12 pieces and roll into balls.
When the time’s up, loosen the stew with a good splash of water if needed, season to perfection, then transfer it to an ovenproof pan and place the balls on top.
Pop the lid on and bake at 190°C/375°F/gas 5 for 30 minutes, or until hot through and the dumplings are fluffy and cooked. Serve with some seasonal greens.