Cut 200g onion into thin half-moons. Cut the remaining 200g into quarters and separate the layers into broad petals. Keep them apart and bring the Curry House Base Sauce to a gentle simmer.
Heat 15ml oil or ghee in a wide 3 litre frying pan over high heat. Add the petals in a loose layer and sear for 3 to 4 minutes, turning occasionally, until charred at the edges but still slightly firm. Transfer them to a plate.
Reduce the heat to medium and add the remaining 25ml oil or ghee. Cook the sliced onions for 10 to 12 minutes, stirring regularly, until soft and richly golden. Add 15ml hot water and scrape the base of the pan.
**Temper the whole spices and aromatics.** Add the cardamom, cumin seeds and crushed coriander seeds. Stir for 30 seconds, then add the Garlic and Ginger Paste and cook for another 45 seconds.
Add the Mixed Powder, ground cumin and Kashmiri chilli powder. Stir for 20 seconds, then add the tomato purée and 30ml hot water. Cook for 60 to 90 seconds, until the tomato darkens slightly and oil begins to show at the edges.
Add 200ml hot Curry House Base Sauce. Increase the heat and let it bubble steadily for 2 to 3 minutes, scraping concentrated sauce from the sides of the pan.
Add the remaining 200ml base sauce, Curry House Pre-Cooked Chicken and salt. Simmer for 4 to 5 minutes, stirring regularly, until the chicken is steaming hot and the gravy coats it. Add up to the remaining 30ml hot water only if needed.
Crush the kasoori methi between your palms and stir it into the curry with the Garam Masala and two-thirds of the coriander. Fold in the seared petals and simmer for 2 minutes.
Remove the pan from the heat, taste and adjust the salt if needed. Rest for 3 minutes and garnish with the remaining coriander. Remove the cardamom pods as they are found when serving.