Trim the lamb of all fat and then slice it into slices around 1.5cm (½") thick, then place in a bowl.
Mash the garlic and throw it in with the lamb, along with the grated ginger.
Pour in the yoghurt followed by the spices, herbs and salt for the tikka. Stir and cover with cling film, then place in the fridge and allow to marinate for between 4 and 24 hours.
If you are using bamboo skewers, soak them for 30 minutes before using them.
Cut the onion for the sauce into a 1.5cm (½") dice.
Cut the chillies in half lengthways.
Heat a 30cm or 12" frying pan over a medium heat and add the ghee. Throw in the curry leaves, slit chillies and onion and cook for 10 minutes.
Pour in the tomato passata and tomato puree then stir to combine.
Add the ground cinnamon, cloves, garam masala, fenugreek leaves, sugar, salt and water. Simmer on a low heat for 15 minutes.
Thread the lamb onto the skewers and then place under a fiercely hot grill or broiler and cook for 2-3 minutes per side.
Scrape any remaining marinade into the sauce and stir to combine.
Transfer the sauce to a blender and blitz until smooth. Pass the sauce through a fine mesh sieve (optional) and back into a pan.
Add the yoghurt for the masala sauce and stir.
Slide the lamb off the skewers and into the sauce, stir and serve.