Set the oven at 440 F (225 C).
Roll out each Filo dough sheet to double its size. Split it down the middle.
Fry the onion in a little butter and oil. This should not change its colour, only make it become soft (its flavour will become a bit milder).
Divide the onion between the sheets of Filo dough and set it all in the oven for about 15 minutes. The dough should rise, become crisp and turn a bit golden in colour.
The Sauce: Peel the apple and slice it thinly. Mix it with the capers and dill in a bowl. Stir in the mayonnaise and crème fraîche. Season with salt and pepper.
Remove the dough from the oven, place the salmon over it and finish it off with dabs of the apple-caper sauce.