Preheat the oven and cook the pasta: Set your oven to 180°C (fan-assisted) or 200°C conventional. Bring a large pot of salted water to a rolling boil—you’ll see big, vigorous bubbles. Add the noodles and cook until al dente, about 8-10 minutes. When you bite a piece, it should have a slight resistance in the centre. Drain well and set aside. You’ll notice the pasta smells softly of wheat and steam rises from the colander.

Sauté the aromatics: In a large, ovenproof skillet or saucepan, melt the 50g butter over a medium heat. Add the diced onion and celery. Stir them with a wooden spoon; they’ll sizzle gently and the onion will turn translucent and fragrant after about 5 minutes. The celery will soften and release a fresh, green scent.

Make the roux: Sprinkle the plain flour over the softened vegetables. Stir constantly for 1 minute—the mixture will look like a thick, pale paste and smell slightly nutty. This step cooks out the raw flour taste, ensuring a smooth sauce.

Build the creamy sauce: Slowly pour in the milk, whisking continuously to prevent lumps. Follow with the chicken stock. Keep whisking over a medium heat until the sauce thickens—you’ll see it change from a thin, watery liquid to a glossy, bubbling sauce that coats the back of a spoon. This takes about 3-4 minutes. Stir in the Dijon mustard, grated cheddar, and a generous pinch of salt and pepper. The cheese will melt into golden ribbons, and the sauce will smell rich and savoury.

Combine everything: Fold in the drained tuna, frozen peas, and the cooked pasta. Use a gentle folding motion—you want the tuna to break into flakes but not turn into mush. The peas will stay bright green and the sauce will cling to every piece of pasta.

Add the topping and bake: In a small bowl, mix the breadcrumbs with the melted butter until they look like damp sand. Sprinkle evenly over the casserole. Place the skillet (or transfer to a baking dish) in the oven. Bake for 20-25 minutes. You’ll know it’s done when the top is deep golden brown and the sauce bubbles up around the edges with a soft hissing sound. Let it rest for 5 minutes before serving—the sauce will settle into a creamy, cohesive blanket.