In a large bowl combine 3 cups all-purpose flour and 1/2 teaspoon salt.
Gradually add 3/4 cup vegetable oil and 1/2 cup cold water to the flour mixture, stirring until a dough forms. Knead briefly in the bowl or on a lightly floured surface until smooth and cohesive (about 1–2 minutes).
Cover the dough and refrigerate for 30 minutes.
While the dough rests, prepare the filling. Heat a skillet over medium heat. Add the 1 large diced onion, 1 red diced bell pepper, and 2 cloves minced garlic to the skillet and cook, stirring occasionally, until the vegetables are soft (about 6–8 minutes).
Stir in 1 teaspoon cumin, 1/2 teaspoon paprika, and 1/2 teaspoon black pepper. Add 1 cup cooked, drained black beans and 1/2 cup cooked corn. Cook the mixture, stirring, for 5 more minutes to heat through.
Remove the skillet from the heat and stir in 1/4 cup chopped fresh cilantro. Transfer the filling to a bowl to cool slightly so it is easier to handle.
Preheat the oven to 375°F (190°C). Prepare a baking sheet.
On a lightly floured surface, roll the chilled dough to about 1/8–1/4 inch thickness. Use a 4-inch round cutter or the rim of a glass to cut circles from the dough.
Spoon about 1 to 2 tablespoons of the filling onto one half of each dough circle, leaving a small border around the edge.
Fold the other half of the circle over the filling to form a half-moon. Press the edges together to seal, then crimp the edges with a fork or by pinching to ensure they are fully closed. Place the sealed empanadas on the prepared baking sheet.
Bake the empanadas in the preheated oven until golden, about 25 minutes. Remove from the oven, let cool slightly, and serve warm.