Heat oven to 425 degrees Fahrenheit.
Cook pasta for two minutes shorter than the package's instructions.
The cooked pasta should be drained and kept aside.
Put the panko, Parmesan, and 2 tablespoons of oil into a small bowl and mix well.
To make a simmering sauce, heat the remaining 2 teaspoons of oil over medium heat.
Add the onion and simmer for about 8 minutes, or until softened, seasoning with salt & pepper to taste.
Stir the flour and onion together after adding them.
The milk and broth should be added while whisking slowly.
Stir the sauce continuously as it starts to boil.
During the 8 to 10 minutes while the sauce is simmering, stir often until the sauce has thickened.
Season to taste with salt and pepper before combining the spaghetti, tuna, and peas.
Additionally, you can add your own seasonings here, such as dried herbs like dried thyme or dried rosemary.
Place the tuna mixture in a prepared 9 by 13-inch baking dish.
Add the panko mixture on top, covering the casserole completely.
Bake until the sauce is simmering and the crust is golden and starting to brown, 20–25 minutes.
Before serving, allow cooling for a short while.
Enjoy after serving.