Mix all the ingredients together and keep stirring to avoid lumps.
Make the mushroom mixture:
Melt the butter add onions and capsicum and stir-fry till they look glossy.
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 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 Oven Temp: 325F-160C. Bake in a pre-heated oven for half an hour, or till cheese is slightly brown.