Ingredients :

• The potatoes — Charlotte or Monalisa, no negotiating on that. These waxy varieties hold up during cooking without turning into mash. Bintje is for fries. Count about 600 g for 4 people — 3 large potatoes.
• The salmon — Smoked salmon is the simplest and works very well. It’s already salted, so go easy on seasoning the béchamel. If you prefer fresh salmon, poach it for 5 minutes in a little boiling water before flaking it — it will be milder and less briny.
• The grated cheese — Emmental for melting, Comté for flavor. A mix of both is ideal. Avoid pre-shredded cheese in bags: it contains anti-caking starch that prevents it from browning well. Grate it yourself; it takes two minutes.
• The butter and flour — Equal parts roux: 50 g butter, 50 g flour. Preferably unsalted butter — not semi-salted — to better control the final seasoning. Ordinary all-purpose flour is perfectly fine.