How to Make It
Preheat the oven to 400°. Line a large baking sheet with parchment paper. Put the potatoes in a medium saucepan and add enough cold water to cover them by 2 inches. Bring the potatoes to a boil over high heat. Add 1 tablespoon of salt, reduce the heat to moderate and boil the potatoes until they are tender, about 25 minutes. Drain the potatoes and let them cool slightly, then peel and cut them into 1/4-inch dice. Transfer to a medium bowl.
Meanwhile, on a lightly floured work surface, roll out the chilled puff pastry 1/8 inch thick. Using a 3-inch round biscuit cutter, cut out 6 rounds. With a 2 1/2-inch round biscuit cutter, make a deep indentation in the center of each round without cutting all the way through the pastry. Set the rounds on the prepared baking sheet. Prick the inside rounds all over with a fork, then brush the pastry rounds all over with the egg wash. Bake the pastry shells for 18 minutes, or until they are browned and puffed. Let the pastry shells cool slightly on the baking sheet. Using a paring knife, cut around and lift out the centers of the pastry shells and reserve.
In a very large bowl, blend the yogurt with 2 tablespoons of the crème fraîche, the water, sliced scallion, minced onion, chopped parsley and vinegar. Fold in the diced potatoes. Season the potato salad with salt and pepper.
Halve the hard-cooked egg. Push the white through a fine sieve set over a small bowl. Repeat with the egg yolk over another small bowl.
Arrange the pastry shells on a large platter. Spoon the potato salad into the pastry shells and top each tartlet with 1 tablespoon of crème fraîche and a small dollop of caviar. Sprinkle the sieved egg white and yolk over the caviar, followed by the minced chives. Top the tartlets with the reserved pastry lids, tipping them on a diagonal so you can see the garnished potato salad. Serve at once with lemon wedges.