Peel one and a half onions and cut into thin half-rings.
Finely chop the remaining half of an onion and mash this together in a bowl with the avocado. The tomatoes, coriander, and jalapeño may now be either blitzed briefly in a hand blender or chopped by hand before being added to the mixture. Season with salt and pepper and add the juice of half of a lemon or thereabouts.
Fry the cod using a tablespoon of olive oil. Start with the skin side down, pressing the fish down into the pan for a moment to prevent the fillet curling up. The flesh should stay juicy and bright.
Put the balsamic vinegar, water, honey, and allspice into a saucepan and season with pepper and around half a teaspoon of salt. Heat until it is beginning to bubble, then take off the heat, add in the onions and leave to cool to room temperature.
Flake the fish with your hands.
Cut the tortillas into circles of around 10cm in diameter. Heat these in a dry pan just enough to warm them through and form them over a wooden spoon or similar into a boat shape such as can later be filled.
Into these boats put first salad, then salsa. Place the flaked cod on top followed by the onions.