1. Preheat oven to 180 degrees Fahrenheit.
2. In a pan on medium low, add the butter and melt.
3. Add the flour into the pan and stir continuously to make a rouge. Turn the heat down lower during this process and let the flour cook down.
4. Add the milk bit by bit while stirring and let the flour absorb it. Make sure there are no lumps.
5. Add the cheese and stir until it dissolves completely.
6. Add more milk if the mix is too thick.
7. Season with salt, pepper, cayenne pepper and nutmeg and mix well.
8. Throw in the chopped capsicum. and celery.
9. Add in the flaked tuna and combine well. Add the mushroom next. Let this simmer for few minutes.
10. Cook the pasta in salted boiling water. Drain.
11. Add the cooked pasta into the pan and mix well.
12. Pour the mix into a buttered baking dish.
13. Sprinkle some grated cheese on the top and pop into the oven until brown.