Clean the fish slices and pat them completely dry using a kitchen towel.
Make a few shallow cuts on thicker fish pieces so the marinade can absorb better.
Combine chilli powder, turmeric, coriander powder, black pepper, ginger-garlic paste, lemon juice, fennel powder, and salt to make a thick masala paste.
Coat each fish slice evenly with the marinade on both sides.
Cover and refrigerate for at least 30 minutes, or up to 4 hours for deeper flavor.
Heat coconut oil in a frying pan over medium heat and add curry leaves.
Place the marinated fish slices in the pan and fry until the coating becomes crisp and the fish is cooked through.
Flip carefully and cook the other side until golden and firm.
Remove from the pan and serve hot with rice or your favorite side dishes.