Marinate the chicken in buttermilk (or yogurt + water) for 3-12 hours in the refrigerator.
Make the spice rub by mixing tandoori masala, brown sugar, black pepper, garam masala, cumin, coriander, kasoori methi, paprika, and salt.
Preheat oven to 400°F (200°C or 375°F with convection fan).
Place the sliced onions in a baking dish or cast iron pot, then add the marinated chicken and spice rub. Mix well.
Bake for 30 minutes, then baste the chicken with paprika-butter mixture and bake for another 30 minutes.
In the last 10 minutes, baste the chicken again with paprika-butter and add whole chillies.
Make a cornstarch slurry with water and add it to the gravy to thicken it, if desired.
Garnish with fresh thyme, parsley, or coriander.
