To make assembly easier, put the cream cheese mixture in a plastic resealable bag. Cut one of the bottom corners of the bag and squeeze the bag to dispense the mixture. The same can be done with the apricot jam.
Preheat the oven to 400 degrees F.
Thaw and cut each puff pastry sheet into 9 squares. In each square, slice the dough a quarter-inch from the edge on all four sides.
Wash the leeks well to remove sand and cut in half lengthwise. Cut the leeks, starting at the white end, into 8 equal slices (approximately 4 cups).
Sauté the leeks and tarragon in the olive oil. Season with salt and pepper and cook until translucent. Let cool.
In a bowl, combine half of the apricot preserves with the cream cheese and mix to combine.
To assemble each pastry, take one square and, working within the score lines, place about 1 tablespoon of the cream cheese mixture, top with 1 to 2 teaspoons of the leeks, 1 teaspoon feta cheese crumbles, and finally 1 teaspoon of the remaining apricot jam.
Bake until golden brown around the edges, 18 to 20 minutes.