To make pickled red onion: place the sliced red onion in a small, wide bottom bowl. Pour red wine vinegar (or white wine vinegar) into the bowl until ¾ of the onion is submerged. Pour water into the bowl until the onions are completely submerged. Set aside and occasionally stir to make sure all the onions get pickled.
Preheat the oven to 425°F. Line a baking sheet with parchment paper, and lightly flour.
Once the puff pastry is thawed, gently open and lay them on the prepared baking sheet, overlapping by 1 inch. Take a rolling pin and apply a bit of pressure to stick the pieces together and roll out the seams so they are smooth. If the edges are uneven you can trim it as needed with a knife.
Next score a line 1 inch from the edge all the way around, being careful not to cut through the dough (use a butter knife). Using a fork, prick the center (inside the score lines) to prevent it from rising.
Whisk the egg and tablespoon water in a small bowl to make an egg wash. Brush the egg wash all over the puff pastry. Sprinkle the everything seasoning all along the border.
Bake in the oven for 12-14 minutes until golden brown. Remove from the oven. If the center puffed up, gently press the center with a back of a spoon to flatten. Let cool.
Meanwhile as the puff pastry is cooking, using a hand mixer or stand mixer, whip the cream cheese for 2-3 minutes. Set aside.
Once the puff pastry has cooled, gently spread the cream cheese evenly over the entire center.
Layer the smoked salmon, then scatter the red onion all over. Garnish with fresh dill and squeeze the lemon. Feel free to sprinkle more everything seasoning over the middle. Serve and enjoy!