Chicken stock is used here so the gratin is not quite so rich, however you could use milk in its place or if you are feeling indulgent, just cream.
Preheat the oven to 180°C.
Rub a gratin dish with the butter.
Thinly slice the potatoes (a mandoline is good to use here).
Layer the potatoes into the gratin dish, scattering over the garlic, thyme and gruyere with each layer. Season with salt and pepper.
Pour over the stock and cream. Bake for 1 1/2 hours or until golden and bubbling
2kg potatoes (weight), peeled
2 cloves garlic, thinly sliced
1 tablespoon thyme leaves
100g gruyere or havarti, grated
200mls chicken, vegetable stock or milk