Ingredients :

• Potatoes — Yukon Gold or Charlotte, nothing else. Floury potatoes like Bintje tend to disintegrate and soak up all the sauce. Yukon Golds stay firm, hold their slice, and their slightly buttery flesh pairs perfectly with the cheese. No need to peel them if they are well washed.
• Gruyère — Buy it in a block and grate it yourself. Pre-shredded bag cheese is often treated with starch to prevent clumping — this prevents melting and gives a grainy texture. A good cave-aged Gruyère, aged at least 6 months, melts into a silky, fragrant ribbon.
• Cheddar — Extra-mature if you can. Young cheddar is too mild to contribute anything here. You need that tangy, almost sharp taste that contrasts with the sweetness of the béchamel and makes the cheese presence felt.
• Mustard powder — The ingredient no one understands the use of until they leave it out. It doesn’t give a mustard taste — it amplifies the cheese, giving it more presence. Half a teaspoon is enough. If you don’t have any, a small spoonful of Dijon mustard added off the heat works very well.