Heat a deep skillet over medium-high heat with the oil. Add butter and melt. When butter foams, add onion, ginger, chili and garlic; season with salt and spices. Sauté to tender 5-6 minutes, add tomatoes and cook 2-3 minutes more. Add to a food processor along with stock and spinach. Purée into a smooth thick sauce, return to pan and bring to bubble. Reduce heat to a simmer and poach chicken in sauce 6-7 minutes until cooked through.
Serve with grilled, buttered Naan bread or prepared rice with cardamom seed alongside.