For the lamb belly filling, preheat the oven to 160C/140C Fan/Gas 3.
Heat a splash of the rapeseed oil in a heavy-bottomed casserole pan and fry the onion, carrot and garlic for a few minutes. Add the lamb belly, brown on both sides and then add the stock, bay leaves, mace, black cardomon and thyme leaves, and cook for around 4–5 hours, or until the lamb pulls away easily from the bone.
For the pierogi dough, combine the flour and salt in a bowl. Make a well and add the soured cream and enough water to form a soft dough. Cover and set aside while the lamb continues to cook.
For the damson chilli jam, put all ingredients into a heavy-based pan with 75ml/2¾fl oz water. Heat over a high heat until the mixture starts to bubble and the damsons to break down. Turn the heat down and gently cook for about half an hour. Set to one side and cool before serving.
To continue the lamb, carefully remove the lamb belly from the pan and pull the lamb meat from the bones. Leave the meat to one side.
Place the cooking liquor back on the heat, add the brown sugar, and cook until the volume the liquid has reduced and the consistency is thickened and will coat the meat.
Mix together the meat and liquor, allow to cool, then add the cooked diced potato.
To shape the pierogi, divide the pierogi dough into thirds. Keeping the extra dough covered, roll each section 3mm/⅛ in thick, adding flour as needed. Cut out 7.5cm/3in circles, saving the leftover scraps of dough.
Fill each circle with about one tablespoon of the filling, fold into a half circle, and pinch the edges tightly. Place apart on a tea towel sprinkled with flour.
Place the pierogis in a pan of boiling salted water, stirring at first to keep them separated, and cook for about three minutes until they rise to the top. Cook for a further 30 seconds to a minute.
Heat a little oil in a pan and fry on each side until golden-brown.
For the topping, melt the butter in a pan and soften the shallot until golden-brown. Add the mushrooms and cook for a couple of minutes.
To serve, place the dumplings onto a serving plate and top with the cobnuts and mushrooms. Serve the jam alongside.