In a pot, add 1 cup/250ml milk & cream and combine with 1 tsp salt, 1/2-1/4 tsp black pepper, 1/4 tsp nutmeg and 1 crushed clove of garlic. Bring to a very gentle simmer and allow to infuse for 5 or so mins, then take out your garlic. Don't allow to rapid bubble or foam, this risks the liquid curdling.
Meanwhile, slice your potatoes into very thin rounds (approx 3-4mm or 1/8"). I find this easiest by slicing a thin layer off the base of the potato in order to stabilize when cutting. You can also use a mandoline.
Grab a baking dish and rub with garlic, then do the same with butter. Pour in a dash of the cream and spread over the bottom of the dish (I find using a brush easiest), then add a layer of potatoes, overlapping slightly as needed to prevent gaps. Continue the layers, ensuring you evenly stack and coat each layer. Finish with a thin layer of the cream.
Pop in the oven at 170C/340F for 1 hour, or until knife tender and lightly browned on the top. This may take slightly longer or less time depending on the depth of your dish, so just be vigilant (see notes).
Once buttery soft and lightly browned, take out and sprinkle with gruyere. Pop back under the grill and broil on high until golden and crispy. Allow to rest for a few mins to allow the potatoes to retain shape, then slice into portions and serve up!