Preheat the oven to 425 degrees.
Roll out the puff pastry so that it's roughly 15 inches long and 10 inches wide (it doesn't have to be exact), then transfer to a parchment lined baking sheet.
Lightly dampen the edges with water, then fold them over about 1 inch to form an outer crust.
Use a very sharp paring knife to trim off just the outer edge of the pastry, being careful to remove as little dough as possible. this will help the crust puff up evenly.
Use a fork to prick the inside of the dough all over, then bake until puffed and golden brown, about 15 minutes. Set aside and let cool.
While the pastry bakes, mix together the cream cheese, sour cream, lemon zest, lemon juice, scallions, parsley and capers in a medium bowl.
Once the pastry shell has cooled completely, spread the cream cheese mixture all over the center. Top the cream cheese mixture with large pieces of smoked salmon, then sprinkle with more scallions. Cut into wedges, then serve.