Slice the sea bass fillet with a sharp knife into thin half-centimeter slices, across the grain. Use truly fresh fish; with ceviche, it is not heated, so quality is everything.
Juice the limes and mix the juice with a generous pinch of salt. Place the fish in a shallow dish and pour the lime juice over it until the slices are just submerged. Place in the refrigerator for fifteen minutes. The acid denatures the proteins, making the fish opaque and firm without any heat involved.
Meanwhile, slice the red onion into paper-thin half rings and the red chili pepper into fine rings. Rinse the onion briefly under cold water to make it less sharp.
Drain most of the lime juice and mix the fish with the onion, red chili pepper and olive oil. Gently fold in the diced avocado so it stays intact. Serve on little gem leaves with plenty of torn coriander. Lean fish meat with the fats from avocado and olive oil makes this a light lunch that is still satisfying.