Preheat the oven to 200 °C and cook the noodles al dente.
In a saucepan, soften the onion and mushrooms in 2 tbsp butter, sprinkle with the flour, cook for 1 minute, then whisk in the milk and cook into a thick sauce; season.
Mix the sauce with the noodles, flaked tuna, peas and hot sauce if using, and spread in an ovenproof dish.
Top with the cheese and the breadcrumbs mixed with the remaining melted butter.