Combine the rhubarb, brown sugar, lemon zest and juice, ginger, salt, and 1⁄4 cup water in a medium saucepan. Bring to a boil over high heat, then reduce the heat to medium-low and simmer for 12 to 15 minutes, or until the rhubarb mixture is thick. Remove from the heat, stir in the vanilla, then pour into a heat-safe bowl to cool completely.

Preheat the oven to 400 F. In a small bowl, combine the beaten egg with 1 tablespoon water to make an egg wash.

Roll out one disc of dough to 1⁄8 inch thick. Use a 6-inch biscuit cutter to stamp out 3 circles. Brush the outer edges with a little egg wash before spooning 2 tablespoons of the cooled rhubarb mixture onto one half of each circle. Fold the unfilled side over the filling and crimp the edges with a fork. Brush the tops of each pie with more of the egg wash.

With a sharp knife, make slits in the center for ventilation and sprinkle each pie with 1⁄2 teaspoon of turbinado sugar. Refrigerate. Repeat with the second disc of pie dough and remaining filling.

Bake for 15 to 18 minutes or until the crust is golden and the filling bubbling. Let the pies cool on the baking sheet for 10 minutes before transferring them to a wire rack.