Preheat the oven to 425°F (220°C). Line a baking sheet with parchment paper.
Place the thawed puff pastry sheet on the prepared baking sheet and use a fork to poke holes all over the surface to prevent excessive rising.
In a small bowl beat 1 large egg with 1 teaspoon water. Brush the entire pastry with the egg wash, then sprinkle the edges with 1 to 2 tablespoons of the everything seasoning.
Bake the pastry for 20–25 minutes until golden brown and puffed. If the center puffs excessively, gently press with a fork to deflate slightly.
Allow the pastry to cool slightly so it sets but is still warm. Using a spatula, gently spread the whipped cream cheese evenly over the tart, taking care not to remove too many flaky layers.
Layer 3–4 ounces of smoked salmon over the cream cheese.
Sprinkle quick pickled onions, fresh dill, and capers across the tart, then add another pinch of everything seasoning and a squeeze of lemon to taste.
Slice the tart and serve immediately.
To make the everything seasoning, stir together dried minced onion, toasted sesame seeds, poppy seeds, dried minced garlic, flaked sea salt and black pepper until combined.