1. Place the rinsed fish in a saucepan. Add the onion, 1 bay leaf, half the peppercorns, and allspice and the salt.
2. Pour over the water and wine and gently bring to a simmer. Cover and poach on the lowest heat for barely 5 minutes until the fish is opaque and feels firm when pressed.
3. Carefully lift out the fish and set aside the poaching liquid. Place the fish and onion in a suitable serving dish. Strain the reserved poaching liquid.
4. Measure half a cup and bring to a boil with the vinegar, remaining bay leaf, peppercorns and allspice. Boil for a few minutes. Allow to cool and pour over the fish. Chill overnight. Add salt to taste. Serve with the red cabbage-and-beetroot slaw.
5. To make the slaw, mix the cabbage, beetroot, radishes and onion with the dressing. Check seasoning. Spoon onto the red butter lettuce leaves to serve.