1. For the chicken tikka: Put the chicken in a bowl and rub in 1¼ teaspoon salt and the lemon juice. Prod the chicken pieces lightly with the tip of a knife and rub the seasoning in again, then set aside for 20 minutes.
2. Add the ginger, garlic, cumin, paprika, chilli powder, cream and garam masala. Mix well, cover and refrigerate for 6-8 hours, though longer will not hurt.
3. For the masala: Pour the sunflower oil into a large, preferably non-stick, lidded pan and set it over a medium-high heat. When the oil is hot, add the onions. Stir and fry until they brown, for 6-7 minutes. Add the ginger and garlic and continue to fry, stirring, for a minute.
4. Add the ground coriander, turmeric, chilli powder and paprika. Stir for 10 seconds, then add a tablespoon of yogurt. Stir and fry until it is absorbed. Add the remaining yogurt in this way, a tablespoon at a time.
5. Add the tomatoes and fry for 3-4 minutes, or until they turn pulpy. Add the stock and ¼ teaspoon salt, and bring to a simmer. Cover, reduce the heat to low, and simmer gently for 15-20 minutes. The sauce should turn thick.
6. Stir in the garam masala and coriander leaves, taste and add more salt if you need it. Shortly before you eat, preheat the grill to its highest setting.
7. Thread the chicken onto 2-4 skewers - the flat, sword-like ones are best. Brush with 3 tablespoons of oil and balance the skewers on the rim of a shallow baking tray, so that the meat is suspended and does not touch the tray. Place about 13cm (5 inches) from the source of heat and grill for 6 minutes on each side, or until lightly browned, cooked through and charred in places.
8. When the chicken skewers are cooked, reheat the sauce and fold in the chicken. Serve immediately.