Quarter the onion and cut into thin strips. Leave out the bacon and stir into the crème fraîche with the fat and 1 egg yolk.
Mix in the onion strips. Season with thyme, salt and pepper.
Cut the puff pastry into 8 squares. Fold the corners slightly to the middle and fold the sides slightly. Spread the crème fraîche mass on top, crumble the goat cheese and spread on top.
Mix 1 egg yolk with milk and brush the dough with it.