1kg/2lb 4oz Maris Piper or King Edward potatoes, peeled and cut into 5cm/2in pieces
1 tbsp crushed chilli flakes
2 tbsp chopped coriander
1 tbsp chopped fresh coriander, plus extra for garnish
6 tbsp garlic paste
5 tbsp ginger paste
2 tsp lemon juice
3 onions, sliced and fried until crisp
6 bay leaves
5 green cardamom pods
1 tsp chaat masala
2 tbsp red chilli powder
3 cinnamon sticks, about 5cm/2in each
2 tsp royal (black) cumin seeds
1 tbsp whole cumin seeds
1 tsp garam masala
pinch saffron
1 tbsp salt
sea salt, to taste
oil, for greasing
100g/3陆oz duck fat or vegetable oil
250ml/9fl oz malt vinegar
2 tbsp melted butter
4 tbsp single cream
500g/1lb 2oz plain yoghurt
2 lamb shoulders, about 1.5kg/3lb 5oz each, trimmed of any surface fat