In a large bowl, combine the all-purpose flour, baking powder, salt, and sugar.
Add the vegetable oil and warm water, mixing until a dough forms.
Knead the dough on a lightly floured surface for about 5 minutes until smooth and elastic.
Cover the dough with a damp cloth and let it rest for at least 30 minutes.
In a large skillet, heat the vegetable oil over medium heat.
Add the chopped onion and minced garlic, and sauté until the onion is translucent, about 5 minutes.
Add the finely chopped fish fillets and cook until the fish is opaque and cooked through.
Stir in the chopped bell pepper, chili pepper (if using), tomato paste, ground cumin, ground coriander, ground black pepper, and salt. Cook for another 2-3 minutes until well combined.
Remove from heat and stir in the chopped fresh parsley or cilantro. Let the filling cool slightly.
Divide the dough into small balls, about the size of a golf ball.
Roll each ball into a thin circle on a lightly floured surface.
Place a spoonful of the filling on one side of each circle, then fold the dough over to form a semi-circle.
Press the edges to seal, using a little water if necessary. You can also use a fork to crimp the edges for a decorative touch.
Heat the vegetable oil in a deep frying pan or pot over medium-high heat until it reaches 350°F (175°C).
Fry the pastels in batches until golden brown and crispy, about 3-4 minutes per side.
Use a slotted spoon to remove the pastels from the oil and drain on paper towels.
Enjoy your meal!