In a pot boil milk with onion, bay leaf and nutmeg powder. Let it simmer for about 10-15 mins to be able to absorb the flavours. Let it cool down.
In a separate pan, melt butter and Sauté plain flour.
Keep the heat really low and constantly keep stirring. When the colour of the flour becomes slightly yellowish switch off the flame. It will become doughy. Let it cool down for couple of minutes.
Switch on the cooker on a very low flame and slowly add the cooled down milk to the pan. Keep stirring with a whisk. This is to avoid any lumps in the sauce. Once it’s thickened enough switch off the flame, add salt, pepper powder and green chilli sauce.
Mix in the prepared chicken mixture. Taste and adjust the seasonings if needed.
Finally, in an ovenproof dish, layer the spinach puree followed by the Bachamel sauce chicken mixture. Top it up with grated cheese. Bake it in a preheated oven at 200 degree Celsius for about 20 minutes. Everything in the dish is cooked this is just to finish off with a crisp golden top of the dish.