Bring a large pot of salted water to a boil. Add spaghetti and cook until al dente, about 8-10 minutes. Drain and set aside.

In a skillet, melt butter over medium heat. Add red onion and sauté for 3-4 minutes until translucent. Add mushrooms and cook for another 5-6 minutes.

Push veggies to one side, add minced garlic and cook for 1 minute. Stir in flour, then gradually whisk in milk until thickened, about 5 minutes.

Fold in drained tuna and peas. Add cooked spaghetti and mix until coated in sauce.

Preheat oven to 350°F (175°C). Transfer mixture to a greased baking dish, sprinkle breadcrumbs, and bake for 15 minutes until golden brown.

Let cool for 5-10 minutes before serving.