Preheat and Prepare: Preheat your oven to 180°C (160°C Fan). Generously grease a 9x13 inch (or equivalent) baking dish with butter or oil.

Cook the Sausage: In a large frying pan over a medium-high heat, cook the sausage meat, breaking it up with a spoon, for 8-10 minutes until browned all over. Use a slotted spoon to transfer the cooked sausage to a plate, leaving the rendered fat in the pan.

Sauté the Onion: Add the chopped onion to the same frying pan and cook in the sausage fat for 5-7 minutes until softened and translucent.

Assemble the Base: Spread the thawed hash browns in an even layer across the bottom of your prepared baking dish. Scatter the cooked sausage and sautéed onions evenly over the hash browns.

Create the Egg Custard: In a large mixing bowl, whisk the 10 eggs until well combined. Whisk in the milk, double cream, Dijon mustard, salt, pepper, and nutmeg until the mixture is smooth.

Combine and Soak: Pour the egg mixture evenly over the layers in the baking dish. Gently press down with a spatula to ensure everything is submerged. What works best for me is letting it sit for 10 minutes at this stage to allow the hash browns to absorb some of the liquid.

Add Cheese: Sprinkle the grated mature cheddar evenly over the top of the casserole.

Bake to Perfection: Cover the dish with aluminium foil and bake for 30 minutes. Then, remove the foil and continue to bake for another 20-25 minutes, or until the casserole is set in the centre and the top is golden brown and bubbling. A knife inserted into the centre should come out clean.

Rest and Garnish: Let the casserole rest for at least 10 minutes before slicing. This is crucial for it to set properly. Garnish with finely sliced spring onions before serving.