In a large bowl, mix 3 cups all-purpose flour with ½ teaspoon salt. Cut in ½ cup chilled, diced unsalted butter until the mixture becomes crumbly. In a separate bowl, whisk together 1 egg, ⅓ cup cold water, and 1 tablespoon vinegar. Gradually add this wet mixture to the flour mixture, kneading gently until a soft dough forms. Shape the dough into a ball, wrap it in plastic, and refrigerate for 1 hour to rest.

Heat 1 tablespoon olive oil in a skillet over medium heat. Sauté the finely chopped onion and minced garlic until softened and fragrant. Add ½ pound ground beef or chicken, cooking until browned and breaking it apart to avoid clumps. Stir in ½ teaspoon cumin and ½ teaspoon paprika along with optional ½ cup diced potatoes or peas. Season with salt and pepper to taste. Remove from heat and let the filling cool.

Preheat your oven to 375°F (190°C). On a floured surface, roll out the dough to about ⅛-inch thickness. Cut the dough into circles approximately 4 to 6 inches in diameter. Place a spoonful of the cooled filling onto each circle. Fold each dough circle in half to form a half-moon, pressing the edges together firmly and sealing them with the tines of a fork to prevent filling leakage.

Arrange the assembled empanadas on a baking sheet. Brush the tops with a beaten egg to help achieve a glossy, golden finish when baking.

Bake the empanadas in the preheated oven for 20-25 minutes or until golden brown and crisp.

Heat oil in a deep skillet or frying pan. Fry empanadas for 2-3 minutes per side until golden and crisp. Drain on paper towels before serving.