Heat the oil in a large casserole dish or cooking pot. Add the onion and cook for 10 minutes until softened and golden.
Add the lamb and cook for 5 minutes until lightly browned.
Stir in the garlic paste, ginger paste and sliced bird's eye chilli. Cook for 1 minute.
Add the curry powder and stir through the tomato paste and cook for 30 seconds.
Add the chopped tomatoes, stock, whole bird's eye chillies and cinnamon stick. Season well with salt and pepper.
Bring to a simmer, cover with a lid and cook gently for 1½ hours, stirring occasionally, until the lamb is tender.
Remove the lid and simmer until the sauce has reduced and is thick enough to coat the lamb.
Remove the cinnamon stick, stir through the coriander and serve with rice and warm naan.