Stir the tomato puree into 120ml hot water until smooth. Blend the cashews with 60ml water into a smooth paste. Whisk the yoghurt, heat the base curry sauce until steaming and place the three measured portions beside the hob.
Heat the ghee or oil in a wide pan with a capacity of at least 3 litres over a medium high heat. Add the Garlic and Ginger Paste and stir for about 30 seconds, until its raw aroma softens without browning.
Reduce the heat to medium. Add the Mixed Powder, paprika, Kashmiri chilli powder, optional hot chilli powder and cumin. Stir continuously for 20 to 30 seconds, just until fragrant.
Pour in the diluted tomato puree and stir thoroughly. Add the first 200ml portion of hot base curry sauce, bring to a lively simmer and cook for 2 minutes, stirring occasionally.
Add the pre-cooked lamb, its measured cooking liquid if using, the cashew paste and the second 200ml portion of base curry sauce. Simmer for 4 minutes, turning the lamb through the sauce.
Add up to the final 100ml portion of base curry sauce as required. Simmer for 4 to 6 minutes, stirring more frequently as the sauce reduces, until the lamb is steaming hot throughout and the sauce coats it without becoming dry.
Turn the heat to low. Add the yoghurt one spoonful at a time, stirring each addition into the sauce. Fold in the tomato wedges and cook gently for 2 minutes without allowing the curry to boil hard.
Stir in the garam masala, crushed dried fenugreek leaves, salt and 15g of the coriander. Taste and adjust the seasoning. Remove from the heat, garnish with the remaining 5g coriander and add the optional sliced red onion before serving.