Ingredients :

• Puff pastry — Take it round, ready to roll out. Brand doesn’t matter, but look for ‘all butter’ — it really makes a difference in crispiness and that slightly buttery taste you’re after. Avoid light versions, they puff much less during baking.
• Yellow onions — The classic ones. Not red, which keep a slight acidity even after cooking, not white which are too mild. Yellow onions caramelize perfectly and develop an almost jammy sweetness when cooked slowly. Two large onions are plenty.
• Crème fraîche (thick) — Thick, not liquid. The liquid version runs everywhere when filling and makes the pastry soggy underneath. With the thick version, the filling stays compact and creamy after baking — exactly what you want.
• Grated cheese — Emmental, Gruyère, Comté — whatever you have in the fridge. Comté gives more character and a slight hazelnut note, Emmental a milder, neutral taste. Both work very well here, no hierarchy.