If you buy the whole fish and not the fillet, do the following: Rinse the fish thoroughly with water. The fish should be uncleaned, meaning with scales and unopened.

Prepare the fish by making an incision at the top and splitting the head in two. Once opened, fillet the skeleton without separating it from the fish, like a book. Use a knife to make a channel in the flesh where you will rub salt, pepper, oil, and lime juice. Place the onion and tomato on top of the flesh or fillet.

For cooking, place the fish or fillet on a griddle or skillet with the scales side down, and cover with banana leaves. Flip after 10 minutes and continue this action until fully cooked.

This flipping action is called 'zarandear', which is where the name comes from. It's necessary to have special tongs to flip it as it's very delicate. Once done, you can serve it with your favorite sauce, pico de gallo, and tortillas as shown in the picture ;)