Wash the clams until clean. Take one in each hand and strike them together; if it makes a dry sound; reject the shell that produced that sound. Place the clams in an aluminum pot and cook over a low flame without adding water.
Stir occasionally until the shells open. When they are all open, and before the juice evaporates, remove from the fire and let them cool off. Remove the clams from its shell and place them in a container. In another container squeeze the lemons and place them with the juice in the refrigerator. If any of the clams remained closed, open them by introducing a knife in to the groove. Cut the black winglets on the sides, cut them into four pieces and place them in the container with the juice.
To prepare the cebiche: finely cut the onions, rinse them in cold water, sprinkle salt and squeeze juice from 4 lemons and let sit. Later add the clams, add the ketchup, a little ají, the cilantro and a couple of drops of oil. Season to your liking.
Serve with toasted maize and bread.