Finishing the dish. Peel the cucumber, turning all the time so you end up with pappardelle-style ribbons (stop when you reach the seeds). Dress the cucumber with a little sousing liquid, some olive oil and seasoning. Mix the horseradish and crème fraîche.
Cut the mackerel into pieces of 4 or 5. Pile cucumber in the centre of each plate and surround with mackerel. Dress the plates with some veg and a little sousing liquid, then put a quenelle [a small scoop] of horseradish cream in the middle and drizzle a little more oil over the fish.