Preheat the oven to 375°F (190°C).

In a mixing bowl, combine the corn masa, vegetable oil, warm water, and salt. Mix until the dough is smooth and easy to handle.

Divide the dough into 8 equal portions, and shape each portion into a ball.

On a flat surface, place a plastic sheet and flatten the ball with a tortilla press or a rolling pin.

Add a tablespoon of refried beans, grated cheese, and seasoned ground beef on one side of the flattened dough.

Add some diced onions, green and red bell peppers, tomatoes, minced garlic, cumin, and black pepper on top of the meat.

Fold the other side of the dough over the filling to create a half-moon shape.

Use a fork to press the edges of the dough together to seal the filling inside.

Repeat the process with the remaining dough and filling.

Place the pastelitos on a baking sheet lined with parchment paper.

Bake the pastelitos for 20-25 minutes, or until they turn golden brown and crispy.

Remove the pastelitos from the oven and let them cool down for a few minutes before serving.