Heat a frying pan and cook mackerel skin side down for two minutes.
Flip fillets and cook for one minute then rest off the heat.
Heat vinegar and sugar together then macerate shallot rings and salt in the mixture.
Toss celery leaves, chervil, watercress, and croutons together.
Reserve two-thirds of the sliced beetroot.
Blend remaining beetroot into a fine purée and pass through a sieve.
Whisk mustard, lemon juice, and olive oil into the purée and season.
Arrange reserved beetroot on plates and top with pickled shallots.
Toss salad with some dressing and place over the beetroot.
Place mackerel on top and drizzle with remaining dressing.