Place the four squares of puff pastry on a baking sheet covered with baking paper. Place the onion confit in the middle.
Place a slice of goat cheese on each square and sprinkle with hazelnut salt. Close each end of the dough.
Brush with beaten egg yolk and sprinkle with hazelnut salt.
Bake in a preheated oven at 200°C for 15 minutes.