Peal and mince the onions ,put them in a non-stick pan with the butter and the olive oil ,add salt & pepper.
Heat at medium flame, and fry them lightly for 15 minutes (they should get golden).
Preheat oven at 220°C / 425°F ; butter the dish and line the puff pastry .
In a salad bowl, beat the egg yolks, then add the heavy cream. Mix with the onions.
Fill the pastry with the onion preparation and bake for 30 minutes.