Prepare the Potatoes and Veggies : Preheat your oven to 180°C (350°F). Boil the sliced potatoes in a pot of water with chicken stock powder for 5–7 minutes until they are tender but still hold their shape. Drain and set aside. Lightly sauté the courgettes and minced garlic in a non-stick pan for 2–3 minutes to enhance their natural flavors.

Make the Cheese Sauce : In a medium saucepan, melt the butter over medium heat. Stir in the plain flour and cook for 1–2 minutes to form a roux. Gradually whisk in the milk, ensuring there are no lumps. Cook the sauce for 3–4 minutes until it thickens. Mix in Dijon mustard, dried dill, and 1 1/4 cups of grated cheddar cheese. Stir until the cheese melts and the sauce becomes smooth.

Assemble the Casserole : In a greased baking dish, spread half of the potato slices as the base layer. Add half of the courgettes, corn, and tuna. Pour half of the cheese sauce evenly over the layer. Repeat with the remaining potatoes, courgettes, corn, and tuna, then top with the remaining cheese sauce.

Bake and Finish : Sprinkle the reserved 1/2 cup of grated cheddar cheese over the top for a golden crust. Bake in the preheated oven for 25–30 minutes or until the cheese is bubbling and slightly golden on top. Let it cool for 5 minutes before serving.