Mix flour and salt in a large bowl. Gradually add cold water and melted butter until the mixture forms a soft dough. Knead for about 5 minutes until smooth, then cover with a damp cloth and let it rest for 30 minutes.

In a skillet over medium heat, sauté chopped onions until they become translucent and fragrant, about 5 minutes. Add your choice of protein or vegetables, cooking until browned. Season generously with salt, pepper, and herbs.

Divide the dough into small balls about the size of a golf ball. On a floured surface, roll each ball into thin circles, roughly 6 inches in diameter.

Place a spoonful of filling on one half of each dough circle. Carefully fold the other half over to create a pocket. Press edges together firmly, using a fork to crimp and seal them tightly.

Preheat your oven to 375°F for baking or heat oil in a deep pan for frying. If baking, arrange empanadas on a lined baking sheet and brush with egg wash. Bake for 20-25 minutes or fry until golden brown.