Bring a large pot of salted water to a boil. Add the pasta and cook until slightly firm. This usually takes about two minutes less than package instructions. Drain the pasta and set it aside.

Preheat your oven to 190°C (375°F). Lightly grease a medium baking dish with oil or butter. This prevents sticking and helps the pasta bake cook evenly in the oven.

Heat butter in a saucepan over medium heat. Add the chopped onion. Cook slowly for about four minutes. Stir often until the onion becomes soft and slightly sweet.

Add flour to the cooked onion. Stir constantly for one minute. Slowly pour in the milk while whisking. Keep stirring until the sauce becomes smooth and slightly thick.

Mix in the mustard, garlic powder, salt, and pepper. Stir well so the flavors combine. Add half of the cheddar cheese. Let it melt gently into the sauce.

Add drained tuna and frozen peas to the sauce. Break the tuna into small chunks using a spoon. Stir carefully so everything mixes without turning mushy.

Place the cooked pasta into the saucepan. Stir slowly so every piece gets coated with the creamy tuna sauce. This step makes the dish rich and flavorful.

Pour the pasta mixture into your prepared baking dish. Spread it evenly with a spoon. Make sure the tuna and pasta are well distributed.

Sprinkle the remaining cheddar cheese and mozzarella on top. Mix breadcrumbs with olive oil. Scatter them over the cheese to create a crunchy topping.

Place the dish in the oven. Bake for about 20–25 minutes. The top should turn golden and bubbly. Let it rest five minutes before serving.