Adapted from Country Living
Butter a medium sized casserole dish and preheat an oven to 175°C.
In a microwaveable bowl add the butter, garlic, milk, and salt. Microwave on high until the milk just starts to bubble, about 3 minutes.
Rinse the potatoes, dry them well then cut into 3-5mm slices with a knife or mandolin.
In the casserole dish place the slices of potatoes in to a slightly overlapped layer.
Top with 1/4 cup of the milk mixture, salt, pepper, and about 1/5 of the cheese. Repeat until you run out of potatoes then pour the additional milk mixture over the top and the leftover cheese on top as well.
Bake until bubbling and a knife easily pierces the gratin, about 40 minutes. Let cool before serving to allow the cheese to set for a clean cut.