Heat the butter in a pan and saute the onions and tomatoes for 5 to 6 minutes.
Add the chicken and the mushrooms and fry for some time. Now add the salt, pepper, chilli powder, mint and lime juice and mix well. Simmer on low heat for 5 minutes then turn off the heat.
Meanwhile, make the white sauce in another pan by mixing the flour and the milk to a smooth paste. Add the butter, salt and pepper and mix well. Cook over low heat stirring all the time until the sauce is thick. Set aside to cool for some time.
When slightly cool add the white sauce and the beaten egg to the chicken and mushroom mixture and mix well.
Transfer to a greased baking dish and bake in a moderate oven (355 degrees) for 35 to 40 minutes.