Infuse the Milk and Cream: Combine 1/2 cup (130 ml) milk and 1/2 cup (130 ml) heavy cream in a saucepan. Add 1 sprig of fresh thyme and 2 crushed garlic cloves (peeled or unpeeled). Heat gently until it comes to a boil, then reduce the heat and let it simmer. Let the mixture infuse until ready to use.

Prepare the Potatoes: Peel 14 oz (400 g) potatoes and place them in water to prevent browning. Using a knife or mandoline, slice them into 1/8-inch (2-3 mm) thick rounds. Pat the slices dry with a clean kitchen towel to remove moisture. Keep the slices on the towel until ready to use.

Assemble the Gratin: Grease a baking dish (aluminum, glass, or porcelain) with 2/3 tsp (3 g) butter. Optionally, rub the dish with one of the used garlic cloves. Arrange the potato slices in the dish, slightly overlapping them. Season with coarse salt (to taste) and pepper. Strain the infused milk and cream mixture, then pour it over the potatoes until just covered.

Bake the Gratin: Place the dish in a preheated oven at 400°F (200°C). Bake for about 1 hour, until the potatoes are tender and the top is golden brown. Optionally, dot with small pieces of butter before baking. For deeper flavor, let the gratin rest overnight and reheat before serving.

Serving: Let cool slightly before serving. For best flavor, prepare the day before and reheat. Serve alongside roasted meats or enjoy as a main dish with a salad.