Prepare the dish – Generously grease the entire baking dish with butter or olive oil, making sure to coat the base and corners well. This prevents sticking and helps create lightly crisp edges as the casserole bakes.

Cook the onion – Heat olive oil in a pan over medium heat and sauté the chopped onion for 4–5 minutes, stirring occasionally, until soft and translucent. Cooking the onion first removes sharpness and builds a deeper, sweeter flavour.

Layer the base – Spread the fully thawed and lightly drained hash browns evenly across the bottom of the baking dish. Press them down gently with a spatula to create an even, compact layer that will hold together after baking.

Add fillings – Scatter the cooked meat and sautéed onions evenly over the hash browns, ensuring each bite has balanced flavour. Sprinkle half of the cheese over this layer so it melts into the casserole rather than just sitting on top.

Mix the custard – In a large bowl, whisk together the eggs, milk, salt, and pepper until completely smooth and slightly frothy. Whisking thoroughly ensures even seasoning and a consistent texture throughout the dish.

Assemble – Slowly pour the egg mixture over the layered ingredients, moving around the dish to distribute it evenly. Lightly press down with a spoon if needed so the custard seeps into the potatoes and fillings.

Top with cheese – Sprinkle the remaining cheese evenly across the surface. This final layer creates a golden, lightly crisp topping once baked.

Bake until set – Place the dish in a preheated oven at 180°C (350°F) and bake for 45–55 minutes. The casserole is ready when the centre is set, no longer jiggly, and the top is lightly golden.

Rest before serving – Remove from the oven and let the casserole rest for about 10 minutes. This allows the eggs to finish setting and makes slicing cleaner and easier.