Place the defrosted puff pastry on a parchment-lined cookie sheet and cut out a large fish shape.
Use a knife to create a 1/2-inch-thick border around the fish without cutting all the way through.
Refrigerate the pastry while the oven preheats.
Preheat the oven to 400F.
Beat the egg and water together in a small bowl.
Brush the pastry with egg wash.
Press the peppercorn into the front area for the fish's eye, if using.
Bake for 20-25 minutes until deeply golden brown, firm, and dry to the touch.
Remove the pastry and let it cool.
Mix the cream cheese, creme fraiche, lemon zest, dill, and smoked salmon in a small bowl.
Season the filling with salt and pepper.
Press down the tart inside the scored border with the back of a spoon.
Spread the salmon filling evenly inside the border.
Top with radishes, snap peas, and extra dill.
Slice and serve.