Potatoes: The best potatoes for a gratin are starchy varieties like Russets or Yukon Golds, which have the right texture to absorb the creamy sauce without becoming mushy.

Cheese: Gruyère is the traditional choice for its excellent melting qualities and subtle sweet and salty flavor that doesn't overpower the dish. However, variations can include cheeses like sharp cheddar for a bolder taste or a combination of Parmesan and mozzarella for a different flavor profile.

Milk and Flour: These form the base of the béchamel sauce, which is seasoned with nutmeg, salt, and pepper to enhance the natural flavors of the other ingredients.

Garlic and Onions: These aromatics are sautéed to form a flavorful foundation that infuses the entire dish with a delicate yet distinct taste.