For the marinade, mix the marinade ingredients together in a large bowl. Add the lamb chops and coat thoroughly. Cover and set aside for 2 hours.
Meanwhile, make the chhweyla sauce, lightly toast the cumin seeds in a small pan, and then remove from the heat once they smell aromatic. Blend the cumin seeds, chillies, garlic, ginger, asafoetida and turmeric in a blender. Add the vegetable oil and a little salt and pepper, and pulse until you have a smooth paste.
Place the meat on barbecue or under a hot grill for 2 minutes on each side, or until golden-brown all over and cooked through. Cut into 2.5cm/1in cubes.
Mix the chhweyla sauce with the cooked meat. Slowly add the mustard oil to the bowl, and toss well so that all the meat is well-coated in the sauce.
For the garnish, heat the oil in a non-stick pan and add the fenugreek seeds. Fry until the seeds begin to colour, then add the garlic slices and fry until light brown, about 1-2 minutes. Pour this garnish over the meat and scatter over the spring onions.