Boil sliced onions in water for 30 seconds, drain, and place in a mixing bowl.
Mix onions with oregano, peppercorns, lime juice, orange juice, and salt; set aside for one hour.
Prepare a barbecue until coals are ash-white.
Rinse and pat dry the sea bass fillets.
Season skin with salt and chipotle powder, then grill skin-side down for 5 minutes.
Flip fillets for 10 seconds, then remove from heat and flake the meat.
Blend avocado, lime juice, mayonnaise, coriander, and chilli till smooth; season and chill.
Heat each tortilla in an oiled pan for 20 seconds per side until soft.
Assemble tortillas with guacamole, fish, pickled onions, and a squeeze of lime.