To skin the haddock fillets, put them skin side down on a board, dip your fingers in salt, then grip the tail end of the skin. Using a sharp knife, flake off the flesh of the fish by working away from you with a sawing action.
Wash the fillets, then pat them dry with kitchen paper. Cut diagonally into thin even strips, and put in a bowl.
Crush the coriander seeds and peppercorns to a fine powder using a mortar and pestle. Mix with the lime juice and salt, then pour over the fish. Cover and chill in the refrigerator for 24 hours, turning the fish occasionally.
The next day, heat the oil in a pan, add the spring onions and fry gently for 5 minutes. Add the tomatoes and Tabasco sauce to taste, and toss together briskly over the heat for 1–2 minutes. Remove from the heat and leave to cool for 20–30 minutes.
To serve, drain the fish from the marinade, discarding the marinade. Combine the fish with the spring onion and tomato mixture and the chopped coriander. Halve the avocado, peel and remove the stone. Slice the flesh widthways. Arrange the slices around the edge of a serving bowl, and pile the ceviche in the centre. Serve chilled.