Grease an ovenproof dish by rubbing the butter all over the inside of it.
Peel the sweet potatoes and slice very thinly (as thin as you possibly can - a mandolin is best for this if you have one).
Warm the cream and milk over a low heat in a saucepan.
Peel and crush the garlic cloves and add to the cream and milk.
Add a pinch of nutmeg and season well with salt and pepper.
Grate the cheddar cheese.
Layering the Sweet Potato Gratin
Start by arranging a layer of the sweet potato slices on the bottom of your dish.
Pour over a quarter of the milk and cream mixture and then sprinkle over a quarter of the cheddar cheese.
Next another layer of potatoes, then more milk and then more cheese so that you have 4 layers of each ingredient in total.
Make sure to finish with a layer of cheddar cheese on the top.
Place the dish into the oven and cook for around 90 mins until the cheese has browned on the top and the sweet potatoes are cooked all the way through. (You can check this by inserting a knife or a skewer through the gratin to check they are soft).
Serve as a side to a roast dinner or even a steak!