Mix ginger and garlic, set half aside. Combine the rest with spices, salt, lemon and yoghurt.
Add chicken, coat well and chill 3 hours or overnight.
Heat half the butter with the oil. Add onion and reserved ginger–garlic, cook 2 minutes.
Add cardamom, cloves and bay leaves, then turmeric, cumin, garam masala, chilli and coriander. Season and cook 5 minutes.
Add cashews, toast briefly, then water to deglaze. Add tomatoes, simmer 20 minutes (add water if needed).
Remove whole spices, blend until smooth.
Return to pan on low heat. Add remaining butter and cream. Season with sugar.
Preheat BBQ to high. Grill chicken 5–6 minutes, turning until nicely charred.
Add chicken to sauce and warm through.
Serve with rice, kasoori methi, and naan.
