Roll out the puff pastry and use a cookie cutter or glass top to cut out small circles, 6-8 inches in diameter.
Add oil to a saute pan over medium heat. Add the onion and cook until translucent, 2-3 minutes. Add the vinegar and cook for an additional minute.
Combine the onions with all other ingredients, except salmon, shrimp and egg yolk, in a medium bowl and mix well, until creamy.
Spoon mixture into each circle, leaving a ½ inch margin around the edges.
Arrange one salmon piece on top of the filling and then place the one piece of shrimp on top of the salmon. Repeat with all pastry circles.
Top with a dollop of filling and brush the edges of the pastry with egg yolk.
Fold the pastry circles in half and pinch between your thumb and fore finger all the way around the edge to seal well and then push the tines of a fork against this outside edge all the way around for final seal.
Brush entire pastry with egg yolk and transfer the pastries with a flat spatula to a cookie sheet.
If you have enough dough to cut out different shapes, it's fun to add fun decoration to the surface of the puff pastry, but make sure to coat the backs with yolk.
Bake in preheated oven for 30 minutes, or until golden brown.