Preheat oven to 350 degrees F. Melt butter in frying pan. Add the grated onions & fry until golden brown. Add the ginger & garlic pastes. Fry for a minute and add the tomato puree.
Add the chopped green chillies, cilantro , salt and the chili powder. Fry for 2-3 minutes.
Turn heat to low and add the cream stirring constantly. Do not let it boil. Cook for a minute & turn off the heat.
Arrange tandoori chicken pieces in an ovenproof dish. Pour the sauce on it. Heat in a preheated oven at 350 degrees F for 20 minutes Serve hot.