1. Boil pasta until al dente, preheat oven to 350°.
2. In medium saucepot, melt 1 tbsp butter and sautee diced onion until translucent, about 5 mins.
3. Add sliced mushrooms and sautee until they have softened and most of their liquid has evaporated.
4. Add 2 tbsp of butter and melt, then add 2 tbsp flour and mix to combine, cook for 1-2 mins until the flour starts to darken slightly.
5. Slowly pour in milk, mixing constantly, until a thick sauce forms.
6. Add tuna, mayo, salt and pepper, and mix everything to thoroughly combine.
7. Add cooked pasta and toss to coat in sauce.
8. In a separate pan, melt 1 tbsp butter and panko, toast until golden brown, approx 5 mins.
9. Add pasta mixture to greased baking dish, sprinkle with shredded cheese and toasted panko.
10. Bake at 350° for 10-15 mins, or until cheese is melted and bubbly.