Slice the onion in Julianne. Chop the cilantro. Mix together with the lemon juice, the aji Amarillo, salt and pepper. Reserve.
Cut the fish into 1x1 inches cubes and marinate it in the lemon juice mixture for between 15 minutes and 2 hours (the less you marinate it, the softer the result). Serve it cold garnishing with a small bouquet of green leaves.