Tuna and Mackerel Salad with Mooli, Lime, Vanilla and Cucumber

• 1 mackerel, about 300 g, filleted and skinned
• 600 g centre-cut sashimi-grade tuna

Check the mackerel fillets for pin bones, removing any you find with tweezers. Put the two mackerel fillets together, boned sides touching and in opposite directions (ie head end next to tail end) and place on a double layer of cling film. Wrap tightly in the cling film and roll on the work surface, holding both ends of the cling film, to get a neat log. Freeze for 1–2 hours until just firm. Th