Ingredients :

• The potatoes — Choose waxy, melting potatoes — Monalisa or Charlotte, not Bintje which will disintegrate into mush. A mandoline gives regular 3-4 mm slices, which really changes the cooking. It’s doable with a knife, but you need to be patient and consistent.
• Canned tuna — Tuna in water, not oil. Oil makes the gratin heavy and gives it a greasy side that covers everything else. In water, the tuna crumbles more easily and lets the cream and cheese speak for themselves. Drain it really well — press down with a fork on the open can over the sink.
• Crème fraîche — Full-fat cream, liquid or semi-thick. Light cream releases water during cooking and dilutes the whole dish. A mixture of cream and milk in equal parts is the perfect balance: rich enough to be indulgent, without being suffocating.
• Grated cheese — Emmental for a stringy crust, mozzarella for more sweetness, cheddar for a bolder taste. What to avoid: pre-packaged industrial grated cheese. It contains anti-caking starch that prevents a true melt and gives a grainy crust.