In a small bowl, toss together nectarines, dark brown sugar, lemon juice, cornstarch, vanilla bean paste, and ground cardamom. Set aside.

Preheat oven to 400° F / 205° C. Line a half-sheet rimmed tray with parchment.

Unroll pastry on a silicone rolling mat or lightly floured countertop. With a rolling pin, roll to approximately 11 x 14 inches.

With a 3 1/2-inch round metal cutter, cut pastry into 12 circles.

Arrange on prepared tray.

Spoon nectarine filling, dividing it evenly amongst the 12 pastry circles.

With a pastry brush, apply a thin strip of beaten egg around the outer edge of each circle.

Fold the pastry over the nectarine filling to create a half-moon shape. With your fingertips, gently seal the edges to prevent the filling from leaking.

With a fork, crimp the sealed edge. (Alternatively, you can use the back of a knife to create a decorative scalloped edge.)

With a pastry brush, brush the top of each nectarine turnover with beaten egg.

Sprinkle liberally with Demerara sugar.

Bake in preheated oven for 16-18 minutes, or until flaky and deeply golden brown, with bits of fruit bubbling around the edges.

Allow to cool briefly before serving. Serve warm. Store leftover turnovers in an airtight container at room temperature for up to two days.