Slice the loaf lengthwise into thin slices. Grind the fish into coarse paste in a food processor. Add the other ingredients and mix. Check the flavour.
Assemble the sandwich cake by spreading fish paste on the bread slices so that the first layer is bread, then filling, then another layer of bread, then filling, and then bread for the top layer. Allow to settle in the fridge for 24 hours before cutting into servings. Decorate with baby lettuce leaves and fish roe.
Serve as a starter for a winter’s day dinner.