Heat a frying pan until hot, add the olive oil and the mackerel fillets, skin side down, and cook for two minutes until the flesh changes colour halfway up the fish.
Flip the fillets over and cook for one more minute then remove from the heat and set aside to rest for a couple of minutes.
Heat the rice wine vinegar and sugar together in a saucepan then add the shallot rings and salt. Leave to macerate for a few minutes.
Meanwhile, toss the celery leaves, chervil (or parsley), watercress and croutons together.
Set aside two-thirds of the beetroot.
Place the remaining beetroot into a food processor and blend to make a fine purée then pass it through a sieve into a bowl.
Add the mustard and lemon juice then whisk in the olive oil and season to taste with salt and freshly ground black pepper.
To serve, lay the reserved beetroot onto the plates and top with the pickled shallots.
Toss the salad with a little of the dressing then place on top of the beetroot slices.
Add the mackerel fillets then drizzle over more of the dressing.