Preheat the oven to 400°F. Let the puff pastry sit at room temperature for 20 minutes to warm up slightly (make sure it is defrosted first if it was frozen). Unroll the puff pastry onto a rimmed baking sheet. Cut the puff pastry into 30 squares. Brush the tops with milk or cream. Bake for 15 to 18 minutes, until puffed and golden brown and cooked through.
While the puff pastry Is baking, cut the salmon into smaller pieces, about 1 x 3 inches each. Blend together the creme fraiche, pesto, and lemon juice in a small bowl.
When the pastry is cooked, place the baking sheet on a cooling rack and use your fingers (wrapped with a paper towel to protect from the hot puff pastry) to gently press down on the warm squares to make them a bit flatter. Let the puff pastry cool to just barely warm or to room temperature.
Top each square with a couple of teaspoons of the creme fraiche mixture, spreading it over the top, but not quite to the edges. Attractively drape a piece of salmon over the crème fraiche on each piece. Top with capers, chives, or dill – you can definitely combine two or three of these as well. Give each tartlet a tiny sprinkling of lemon zest.
Arrange the tartlets on a platter and serve.
Notes
You can make the crème fraiche spread up to 2 days ahead of time, and you can make the puff pastry squares earlier in the day and leave them at room temperature until it is time to serve them. Assemble just before serving or the puff pastry will get soggy. And tell your friends to bring over some bubbly.