Preheat your oven to 180 C. Grease a 9x13 baking dish with butter.
In a large (I may use a medium size pot for this) non stick pan melt the 1/4 cup of butter then add the onion, celery, bell pepper, and garlic for about 3 minutes, then sprinkle in the flour and mix and cook for another minute. Remove from heat.
Slowly stir in the milk, then return to heat and stir until thickened and the mixture comes to a boil, season with salt and pepper to your liking, stir, then stir in the mushrooms, Cheddar cheese, and tuna, when mixed, add in the egg noodles and mix to combine.
Pour the mixture into the prepared baking dish and smooth out, sprinkle on the breadcrumbs then Parmesan cheese, drizzle with the melted butter.
Bake for 25-30 minutes, remove and let rest 5 minutes, serve.