Grind together the onions, ginger and garlic. Mix into yogurt together with salt, chilli powder, cashew nuts and half the ground coriander, cumin and garam masala. Mix into the chicken and refrigerate for 6 hours. Place the chicken in a heavy-based pan and cook till tender and all the moisture has dried. In separate pan heat ghee or butter to smoking hot and pour over the cooked chicken. Fry to a rich golden brown. When serving put chicken in a serving dish, squeeze the juice of both limes and sprinkle remaining spices over chicken. Garnish with tomato slices, onion rings, coriander leaves and green chillies.