In a saucepan, combine milk, cream, garlic, nutmeg, and thyme. Heat gently until simmering.
Peel and thinly slice potatoes (a mandoline helps). Add to the warm milk mixture.
Simmer for 6-7 minutes, stirring gently.
Grease a baking dish with butter. Layer the potatoes, sprinkling a little hard cheese between layers.
Repeat until all potatoes are used, then top with remaining hard cheese and Gruyère.
Bake at 180°C (350°F) for 25-30 minutes until golden and bubbly.
