Whisk together the oat flour, millet flour, sweet rice flour, tapioca starch, sugar, xanthan gum, and salt in a large bowl.

Add the cold butter and mix with your hands, breaking up the butter until the largest pieces are about the size of a pea.

One tablespoon at a time, dribble in the ice water, combining each time, until the dough holds together when pinched with your fingers (add water until it just holds together, but is not sticky. If you need more water, add a teaspoon at a time).

Roll the dough in two balls and flatten into two discs. Wrap with plastic and refrigerate for 30 minutes.

Preheat the oven to 400ºF.

Sprinkle some millet flour over wax or parchment paper, and roll out the dough over the paper until it’s about ⅛-¼-inch thick. Use a 2-inch for the bottoms and 2½-inch round cutter for the tops to cut out an even number of tops and bottoms of your hand pies, rerolling the dough with millet flour as needed. Store the rounds in the refrigerator on parchment paper squares while you roll out the rest. You should have 10-12 rounds of each size.

Slice small Xs in the top of the larger 2½-inch rounds of pie dough (or punch out a small shape).

Pulse the sugar and rosemary together in a food processor until fine. Set the rosemary sugar aside.

Heat the peaches in a medium skillet with 2 tablespoons of rosemary sugar and tapioca starch over medium heat until they just begin to soften. Transfer to a bowl and refrigerate until no longer warm.

Mix the grated white chocolate together with the mascarpone and set aside.

Place a smaller 2-inch round of pie dough down. Spread a small spoonful of mascarpone over the top of the pie round, leaving ⅛-inch edge around the outside. Place heaping tablespoon of cooked peaches on top of the mascarpone. Lay a larger 2½-inch round of pie dough over the peaches and gently press the edges down. Press a fork into the outside edges to seal. Transfer your hand pie to a parchment-lined baking sheet and repeat with remaining dough.

Gently brush the tops of the hand pies with the egg wash and sprinkle the remaining rosemary sugar over the top (be careful not to let it get all over your pan or it will burn while you bake your pies).

Bake your pies for 30-35 minutes, until the tops are golden brown. Remove from heat and transfer to a wire rack to cool.