First, preheat your oven to 180°C (160°C fan/Gas Mark 4). Lightly grease a 9x13 inch baking dish with butter or cooking spray. You'll want a dish that's ready to embrace all that deliciousness.

In a large mixing bowl, combine the thawed and well-drained hash browns, the finely chopped yellow onion, and half of the melted butter. Use your hands or a sturdy spoon to ensure everything is evenly coated; the potatoes should glisten slightly from the butter.

In a separate medium-sized bowl, whisk together the full-fat milk, chicken or vegetable stock, sour cream, garlic powder, onion powder, black pepper, and sea salt. Continue whisking until the mixture is smooth and well combined, with a creamy, pale yellow hue.

Now, add the grated mature Cheddar and Monterey Jack cheeses to the hash brown mixture. Pour the creamy liquid mixture over the top. Gently fold everything together until the cheese and liquid are evenly distributed throughout the potatoes, making sure every strand of hash brown is coated in that rich, savoury sauce.

Tip the entire hash brown mixture into your prepared baking dish. Spread it out evenly with the back of a spoon or spatula, ensuring the surface is level. Drizzle the remaining melted butter over the top of the casserole, which will help create a beautiful golden crust. If you're using smoked paprika, sprinkle a pinch over the top now for an extra layer of flavour and a lovely reddish tint.

Bake for 45-55 minutes, or until the top is a deep golden brown and you can see the cheese bubbling enticingly around the edges. When it's ready, the kitchen will be filled with a wonderful savoury aroma, and the casserole should feel firm to the touch in the centre.

Once baked, remove the casserole from the oven. Let it rest for at least 10-15 minutes before serving. This crucial resting time allows the cheese sauce to set properly, ensuring that when you cut into it, you get clean, firm slices rather than a runny mess. Garnish with fresh chopped parsley if you wish, adding a pop of green and a fresh scent.