If using thawed or canned fish, drain it thoroughly and press out the excess liquid with a clean dish towel or paper towels. Pat fresh fish dry.
Put the fish and onion in a food processor. Pulse until finely chopped and combined into a coarse paste. If you don’t have a food processor, chop both ingredients very finely.
Transfer the mixture to a bowl. Stir in the egg whites, red chili, lime juice, salt and black pepper.
Heat the oil in a large skillet (frying pan) over medium-high heat.
Add heaping tablespoons of the fish mixture to the skillet. Flatten each portion with the back of a wooden spoon to form 6 to 8 fish cakes.
Cook for about 5 minutes, or until the first side is browned and the fish cakes lift cleanly from the skillet.
Flip the fish cakes and cook for about 5 minutes more. Check that the centers have reached 160°F (71°C).
Serve hot with your choice of sauce and vegetables.