Ingredients :

• Potatoes — Go for Yukon Gold or Charlotte — they hold up during cooking without turning into mash, but stay meltingly soft inside. Russets tend to fall apart: if that’s all you have, cut them thicker, 1.5 cm minimum. Uniform thickness is what guarantees even cooking.
• Yellow Onions — Large, firm, with very dry skin. Not red — too sweet, they disappear. Not white — too pungent, they don’t melt the same way. Yellow onions have exactly the right sugar content to caramelize without burning. Cut them into thick half-moons, about 1 cm — they should keep a bit of bite at the end of cooking.
• Butter — Real semi-salted butter is preferred — it avoids having to salt separately and gives a more complex flavor base. Cut it into small pieces that you slip between the layers: it melts by seeping everywhere rather than staying on the surface. If you want a more rustic taste, clarified butter also works very well.