In a large pot, bring water to a rolling boil and generously salt it. Add the spaghetti and cook according to the package instructions until al dente.

In a deep skillet over medium heat, melt the butter. Add the finely diced red onion and sauté until soft and translucent.

Toss in the sliced mushrooms and stir occasionally until golden brown and fragrant.

Stir in the minced garlic and cook until aromatic. Sprinkle in the flour and whisk for 1 minute.

Gradually pour in the milk while whisking and cook until thickened and smooth.

Gently fold in the drained tuna and frozen peas into the creamy sauce.

Combine the cooked spaghetti with the sauce, ensuring every strand is coated.

Preheat the oven to 350°F (175°C). Transfer the mixture into a greased baking dish and sprinkle breadcrumbs on top.

Bake for 15-20 minutes until the topping is golden brown and crispy.

Let cool for 5-10 minutes before serving.