Make the mushroom mixture:
Melt the butter and add onions and stir-fry till they look wet.
Add the mushrooms and continue to stir-fry over high heat till vegetables look like they are coated with the butter.
Take it off the heat and mix in the spinach, salt, pepper and white sauce.
Transfer mixture into a oven proof serving dish.
Sprinkle the cheese to cover the mixture, garnish with the tomato slices and bake in a pre-heated oven for half an hour, or till cheese is slightly brown.
For the white sauce:
In a heavy based pan, heat the butter, stir in the flour and saute till slightly darkened.
Add milk very gradually, stirring vigorously all the time to avoid lumps. Bring to a boil.
Lower the heat and simmer for 2-3 minutes, continuing to stir to avoid scorching.
Add the salt, pepper and any other seasoning that you desire.
Chicken can be baked in the same way, by using cooked chicken instead of mushrooms, and you need not have the spinach in it.
Key Ingredients: Mushroom, butter, onion, spinach, salt, black pepper, tomato, milk, all purpose flour, nutmeg