Preheat and Prepare: First, preheat your oven to 180°C (160°C fan). Lightly grease a 2-litre casserole or baking dish.

Cook the Noodles: Bring a large pot of salted water to a boil. Add the egg noodles and cook according to package directions, but for 2 minutes less than the recommended time. They will finish cooking in the oven. Drain well and set aside.

Sauté the Aromatics: While the noodles are cooking, heat the olive oil in a large saucepan or Dutch oven over medium heat. Add the finely diced onion and cook for 5-7 minutes, until softened and translucent. Add the minced garlic and cook for another minute until fragrant.

Make the Roux: Add the 60g of unsalted butter to the saucepan with the onions. Once melted, stir in the plain flour to form a thick paste (this is your roux). Cook, stirring constantly, for 1-2 minutes. This step is crucial to cook out the raw flour taste.

Create the Sauce: Gradually whisk in the room-temperature milk, a little at a time, ensuring each addition is fully incorporated before adding the next. This prevents lumps. Continue to cook, whisking often, for about 5-8 minutes, until the sauce has thickened enough to coat the back of a spoon.

Add the Flavour: Remove the saucepan from the heat. Stir in the grated cheddar cheese until it's completely melted and the sauce is smooth. What works best for me is adding the cheese in two batches to ensure it melts evenly. Stir in the Dijon mustard, nutmeg, salt, and a generous amount of black pepper.

Combine Everything: Gently fold the cooked noodles, drained tuna, and frozen peas into the cheese sauce until everything is evenly coated. Pour the mixture into your prepared baking dish and spread it out evenly.

Prepare the Topping: In a small bowl, mix the panko breadcrumbs, grated Parmesan, and melted butter together until the breadcrumbs are lightly coated. Sprinkle this mixture evenly over the top of the casserole.

Bake to Perfection: Place the dish in the preheated oven and bake for 20-25 minutes, or until the top is golden brown and the sauce is bubbling around the edges.

Rest and Serve: Let the casserole rest for 5-10 minutes before serving. This allows the sauce to set slightly, making it easier to serve.