Bring a small pan of water to the boil, add the fenugreek leaves, and blanch for a few minutes, or until soft then drain. Transfer the fenugreek leaves to a mortar or blender and pound with a pestle or process to make a smooth puree.
Heat the mustard oil in a frying pan (skillet) over low-medium heat, add the fenugreek seeds, and stir-fry for 30 seconds, or until they start to splutter, then add the fenugreek puree and cook for about 10 minutes. Remove from the heat and allow to cool.
Put the ginger and half the garlic paste with 1 tablespoon lemon juice into a large shallow dish. Season with salt and mix well. Add the chicken breasts and rub the mixture evenly over to coat. Cover and allow to marinate in the refrigerator for about 1 hour.
Put the yoghurt in another large shallow dish, add the remaining garlic paste, ground dried fenugreek, chillies, ginger, chilli powder, cooked fenugreek puree, remaining lemon juice, gram flour and oil and mix well. Adjust the seasoning, if necessary. Add the chicken breasts and rub the mixture evenly over to coat. Cover and allow to marinate for a further 2 hours.
Prepare a tandoor or charcoal grill for moderate heat, or alternatively preheat the grill (broiler) to medium. Thread the chicken breasts onto several metal skewers and place a tray underneath to collect any juices.
Roast in a moderately hot tandoor, over a charcoal grill or under the grill for 5 minutes, or until half done.
Remove the skewers from the heat and hang them up for about 2-3 minutes to allow any excess moisture to drain out completely. Put the skewers in a dish, baste with melted butter, and roast again in the tandoor, on the charcoal grill, or under the grill for a further 3-4 minutes, or until cooked.
Transfer the chicken to a plate and serve hot