Place the thinly sliced red onion, the apple cider vinegar, and salt in a small bowl. Stir to coat the red onion in the vinegar.

Let the onions soak in the vinegar mixture, stirring a couple times to ensure the pieces stay submerged. Set aside.

Preheat the oven to 425° F and line two large baking trays with parchment paper.

In a small bowl, mix the egg with a splash of water until the yolk is evenly mixed with the egg whites.

Arrange one thawed puff pastry onto each baking tray and use the tines of a fork to poke holes across the entire sheet. Use a pastry brush to brush the egg wash across each sheet of puff pastry.

Sprinkle the everything seasoning along the edges of the puff pastry dough. Bake the pastry until golden brown, according to the package directions (mine took about 15 minutes). 

Add the cream cheese and heavy cream to a large mixing bowl. Use a hand mixer to whip the cream cheese and heavy cream together, until it becomes fluffier. 

Gently mix the fresh dill, fresh chives, lemon zest, lemon juice, and a pinch of salt and pepper into the cream cheese. Taste and adjust the seasoning to your liking and set aside at room temperature.

Once the puff pastry is out of the oven, set it aside for 5-10 minutes to cool slightly. Use a silicone spatula or the back of a spoon to smooth the herbed cream cheese across the center of the pastry. 

Arrange the slices of smoked salmon, sliced cucumber, and tomato wedges across the pastry. Drain the pickled red onions and arrange them across the pastry. Sprinkle the capers across the top of the pastry.

Garnish with additional fresh herbs, if desired. Slice each pastry into 9 equal pieces and serve immediately.