Preheat the oven to 350°F. Line a baking sheet with parchment paper.

Dust a work surface with brown rice flour. Roll out the dough 1/8 inch thick. Using a 3-1/2-inch diameter cookie cutter or glass, cut 16 rounds out of the dough (if needed, reroll the dough 1/8 inch thick and cut out more rounds). Transfer the dough rounds to a parchment paper-lined plate, stacking them between sheets of parchment, and reserve in the refrigerator while making the filling.

In a large bowl, combine the blueberries, sugar, cornstarch, thyme, lemon zest and juice, and salt.

Transfer 8 of the dough rounds to the prepared baking sheet and brush their edges with water. Place a heaping tablespoonful of the blueberry mixture in each center, place another dough round on top of each, and crimp the edge with a fork. Brush the top of each hand pie with the beaten egg, sprinkle with some coarse sugar, and cut a small slit on the top.

Bake until the filling is bubbling and the crust is browned and crisp, about 40 minutes. Cool for 20 minutes before serving.