Put the saffron in a heatproof cup and pour over 75ml boiling water. Leave for 30 mins, or until the water is a deep yellow colour.
Heat the oil in a heavy-bottomed saucepan, and brown the lamb all over – you’ll need to do this in batches. Transfer the browned lamb to a bowl using a slotted spoon as you continue cooking the rest.
Fry the onions in the residual fat in the pan over a medium-low heat for 10-12 mins until golden and soft. Add the tomatoes and garlic, and continue to fry until the tomatoes have softened. Stir in the spices and cook for a few minutes, then return the lamb to the pan with some seasoning. Pour in the stock and infused saffron water. Bring to just under the boil, then add the apricots, chestnuts, and a little more seasoning. Reduce the heat to a simmer, cover and cook over a very low heat for 1 hr-1 hr 15 mins until the lamb is tender. At this stage, the lamb can be frozen. Leave to cool completely first, then transfer to a freezer proof airtight container. Will keep in the freezer for up to three months.
To serve straightaway, stir in the lemon juice, sumac and coriander. Alternatively, if you have frozen the lamb, defrost in the fridge overnight, then transfer to a saucepan and heat gently. When it is piping hot, add the lemon juice, sumac and coriander. Taste to check for seasoning, then serve.