In a large mixing bowl, combine gluten-free all-purpose flour and salt, whisking until blended. Cut in the cold, cubed butter using a pastry cutter until the mixture resembles coarse crumbs. Add a beaten egg and gradually pour in cold water, mixing until a cohesive dough forms. Wrap the dough in plastic wrap and refrigerate for 30 minutes.

While the dough chills, heat a skillet over medium heat. Sauté the finely chopped onions and diced bell pepper in a splash of oil until softened, about 5 minutes. Stir in the cooked, shredded chicken along with cumin (if using), mixing until well combined. Remove from heat, fold in cheese until melted, then let cool.

Preheat your oven to 375°F (190°C) and line a baking sheet with parchment paper. Roll out the dough to maximize your time in the kitchen.

On a lightly floured surface, roll out the chilled dough to about 1/8 inch thick. Cut out circles for the empanadas. Place a spoonful of cooled filling on one half of each circle, fold over, and press edges to seal. Use a fork to crimp edges.

Beat an egg and brush over the top of each empanada. Bake for 20-25 minutes until tops are golden and flaky. Optionally, fry them in hot oil instead of baking.

Let the empanadas cool for a few minutes. Serve warm, perhaps with a side of salsa verde or chimichurri sauce.