Special equipment: a 6-inch round cutter
For the crostatas: Preheat the oven to 400 degrees F. Line a baking sheet with parchment.
In a medium mixing bowl, combine the peaches, rhubarb, lime juice, triple sec, vanilla and 1/3 cup of the sugar.
Unfold the puff pastry dough onto a floured work surface. Using a floured rolling pin, roll out to a thickness of 1/8 inch. Using a round cutter or a 6-inch circular template, cut 4 circles into the dough and transfer to the prepared baking sheet. Fold 1/2 inch of the dough over itself all around the edge of each circle, crimping it with your fingers to create a thin border. Using a fork, pierce the pastry dough all over.
Using a slotted spoon to leave excess liquid behind, spoon some of the fruit mixture into the center of each circle. Using a pastry brush, brush each pastry edge with some of the remaining fruit liquid and sprinkle with the remaining 1 tablespoon sugar. Bake until the edges are golden brown and the centers are bubbling, 25 to 30 minutes.
While the crostatas are baking, reduce the remaining fruit liquid in a small pot to a syrupy consistency, about 5 minutes.
For the creme fraiche: In a small bowl, whisk together the creme fraiche, agave and lime zest and juice until soft peaks form.
Remove the crostatas from the oven and spoon the fruit syrup over the top as a glaze. Allow to cool slightly before serving with a dollop of the creme fraiche and a sprig of mint.