
Whizz the onions, garlic, ginger, and 200ml water in a food processor until you have a smooth paste.
Heat the oil in a casserole dish and fry the onion mixture for 10-15 minutes or until all the liquid has been absorbed and the onions begin to turn golden brown. Add the curry paste and fry for 1 minute more. Stir through the chopped tomatoes, cinnamon stick, and red chilli. Pour in the lamb stock and bring to a simmer. Add the leftover roast lamb and cook, covered, for 1 hour.
Remove the lid and simmer uncovered for 15 minutes to reduce slightly. Season to taste and stir through the coriander. Serve with fluffy basmati rice, naan bread, and yogurt.