Preheat your oven to 375°F (190°C). Lightly grease a 9x13 inch (or similar 2-quart) baking dish.

Cook the pasta in a large pot of salted boiling water according to package directions until al dente (usually 1-2 minutes less than the recommended time, as it will continue to cook in the oven). Add the frozen peas to the pot during the last minute of cooking. Drain well and set aside.

While the pasta cooks, make the cheese sauce. In a large saucepan or skillet, heat the olive oil over medium heat. Add the chopped onion and cook until softened, about 5-6 minutes. Add the minced garlic and cook for another minute until fragrant.

Melt the 4 tbsp of butter in the saucepan with the onions. Once melted, whisk in the flour and cook for 1-2 minutes, stirring constantly, to form a roux.

Gradually pour in the warm milk while whisking continuously to prevent lumps. Bring the sauce to a gentle simmer and cook for 3-5 minutes, until it has thickened enough to coat the back of a spoon.

Remove the saucepan from the heat. Stir in 1.5 cups of the grated cheddar cheese, the Dijon mustard, salt, and pepper. Continue stirring until the cheese is completely melted and the sauce is smooth.

In a large mixing bowl, combine the cooked pasta and peas, the well-drained flaked tuna, and the frozen sweetcorn. Pour the cheese sauce over the top and gently stir everything together until well combined.

Transfer the mixture to the prepared baking dish and spread it into an even layer.

In a small bowl, prepare the topping. Mix together the panko breadcrumbs, grated Parmesan cheese, and the 2 tbsp of melted butter until the breadcrumbs are evenly coated.

Sprinkle the breadcrumb mixture evenly over the pasta, followed by the remaining 1/2 cup of cheddar cheese.

Bake for 20-25 minutes, or until the sauce is bubbly and the topping is golden brown and crispy.

Let the tuna pasta bake rest for 5-10 minutes before serving. This allows the sauce to set slightly. Garnish with fresh parsley if desired.