Preheat the oven to 160C/Gas 4/fan oven 140C. Rub the butter all over the surface of a gratin dish, about 18x28cm/7x11in.
Pour the milk and cream into a saucepan. Add the garlic and thyme sprigs.
Very slowly heat the milk allowing the herbs and garlic to infuse nicely. While this is warming, peel and slice the potatoes to a width of 3mm/1⁄8in, a mandolin is perfect for this. If you only have a knife, then that’s fine, it just takes a little longer! Place the sliced potatoes into a large bowl.
When the cream mixture is too hot to touch but not boiling, add the salt and about 20 twists of black pepper and stir to dissolve. Pour it carefully over the potatoes and mix well with a spoon. You want be sure all the potato slices are coated.
Carefully place half the potato mixture into the buttered dish, trying to keep it as level as possible.
At this point you can add some cheese if you wish, alternatively leave it for the top so that you get that bubbling golden cheesy top when it’s served. Lay the rest of the potato mixture on top and level out as best you can, and sprinkle with your cheese of choice.
Cover the tray with two sheets of aluminum foil and seal it tightly with your hands and bake for about 45 minutes, or until the potatoes are tender when poked with a knife or skewer. Remove the foil and place the tray back in the oven but TURN IT UP HIGH and cook for 10 minutes or until it begins to bubble and go a beautiful golden colour.
When you’re happy, remove the gratin and leave the dish to stand for 5-10 minutes, and then serve.
The amount of cheese you can add is completely up to you and different herbs can be added, both fresh or dried. You could even add in some smoked ham, or crispy bacon to take it to add a further depth to the flavour.
If you have any left, this can be eaten cold/room temperature, and works really well in a sandwich, with butter.