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1
Preheat the oven to 400.
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2
Line a large baking sheet with parchment paper.
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3
Put the potatoes in a medium saucepan and add enough cold water to cover them by 2 inches.
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4
Bring the potatoes to a boil over high heat.
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5
Add 1 tablespoon of salt, reduce the heat to moderate and boil the potatoes until they are tender, about 25 minutes.
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6
Drain the potatoes and let them cool slightly, then peel and cut them into 1/4-inch dice.
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7
Transfer to a medium bowl.
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8
Meanwhile, on a lightly floured work surface, roll out the chilled puff pastry 1/8 inch thick.
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9
Using a 3-inch round biscuit cutter, cut out 6 rounds.
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10
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.
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11
Set the rounds on the prepared baking sheet.
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12
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.
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13
Let the pastry shells cool slightly on the baking sheet.
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14
Using a paring knife, cut around and lift out the centers of the pastry shells and reserve.
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15
In a very large bowl, blend the yogurt with 2 tablespoons of the creme fraiche, the water, sliced scallion, minced onion, chopped parsley and vinegar.
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16
Fold in the diced potatoes.
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17
Season the potato salad with salt and pepper.
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18
Halve the hard-cooked egg.
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19
Push the white through a fine sieve set over a small bowl.
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20
Repeat with the egg yolk over another small bowl.
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21
Arrange the pastry shells on a large platter.
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22
Spoon the potato salad into the pastry shells and top each tartlet with 1 tablespoon of creme fraiche and a small dollop of caviar.
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23
Sprinkle the sieved egg white and yolk over the caviar, followed by the minced chives.
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24
Top the tartlets with the reserved pastry lids, tipping them on a diagonal so you can see the garnished potato salad.
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25
Serve at once with lemon wedges.
