First off, preheat your oven to 180 degrees Celsius. Get ready for a treat!
Place your thinly sliced potatoes into your casserole dish. Don't be shy to make a beautiful arrangement, layer them up nicely!
In a mixing bowl, combine your finely chopped onion, minced garlic, cream, milk, dried rosemary, salt and pepper. Give it a good mix, mates!
Pour this creamy mixture over your layered potatoes in the casserole dish. Ensure the potatoes are well covered.
Sprinkle the grated cheddar cheese on top. Because who doesn't love more cheese, right?
Pop your dish into the preheated oven and bake for about 1 hour, or until the potatoes are soft and the top is golden and bubbly.
Take the dish out of the oven and let it cool for a few minutes before serving.