Gather the ingredients. Preheat the oven to 350 F (175 C).
Peel, rinse, and dry the potatoes and slice as thinly as possible (a mandoline is good for this, but a sharp knife works too).
In a large saucepan, place the potatoes, milk, cream, 1 of the crushed garlic cloves, salt to taste, a dash of pepper, and a dash of nutmeg or mace.
Bring to a very gentle boil over medium-low heat and simmer for 10 minutes until the cream thickens.
Remove from the heat and add the grated Gruyère. Stir until the cheese has melted.
Generously grease a 1 ½ to 2-quart baking dish with 1 teaspoon of the butter, then rub the dish all over with the remaining lightly crushed clove of garlic.
Pour the potato mixture into the dish, using a spoon to gently move the potatoes in the cream mixture to make sure they are all covered.
Finely slice the brie into long strips and lay across the top of the gratin. Dot the surface with the remaining 2 tablespoons butter. Season with salt and pepper.
Bake for about 40 minutes or until the top is bubbly and golden brown and the potatoes are soft when pierced with a knife. If the top is browning too quickly before the potatoes are cooked, cover with aluminum foil. Serve immediately. Enjoy.
