Preheat the oven to 140°C and butter a 6–7 inch casserole or gratin dish.
Arrange one-third of the sliced potatoes in the base of the dish.
Season with salt and pepper, then add one-third of the garlic, thyme, and Gruyère.
Repeat the layering process twice more.
Warm the beef stock, then slowly pour it around the edge of the dish to keep the layers intact.
Bake for 2 hours, or until the top is caramelised and potatoes are cooked through.
