In a small bowl, combine the flour and salt. Add the cold butter and use your fingers or a pastry cutter to work it into the flour until the mixture resembles coarse crumbs.
Gradually pour in cold water, a little at a time, mixing gently until the dough begins to come together. Knead briefly on a floured surface until smooth, then wrap in plastic wrap and chill for 15 minutes to relax the gluten.
While the dough chills, heat olive oil in a skillet over medium heat. Sauté the chopped onions until golden and fragrant, about 5 minutes. Let them cool slightly.
In a mixing bowl, combine the cooked shredded meat, sautéed onions, cumin, smoked paprika, salt, and pepper. Mix well to evenly distribute the spices and flavors.
Preheat your oven to 200°C (390°F) and line a baking sheet with parchment paper. Remove the dough from the fridge and roll it out on a floured surface to about 1/8 inch thick.
Use a round cutter or glass about 3 inches in diameter to cut out 12 circles from the dough. Gather the scraps and reroll if needed to make more circles.
Place a spoonful of filling in the center of each dough circle. Fold the dough over to form a half-moon shape, then pinch or crimp the edges firmly to seal in the filling.
Arrange the filled empanadas on the prepared baking sheet. Brush the tops with beaten egg mixed with a teaspoon of water for a shiny, golden finish.
Bake in the preheated oven for 20-25 minutes, until the empanadas are golden brown and crispy, with a crackling sound when tapped.
Remove from the oven and let rest for 5 minutes. Serve warm, optionally garnished with chopped herbs or a squeeze of lime.
