Stir the lemon juice and two tablespoons of sugar in a small saucepan until the latter starts to dissolve.

Mix in the rhubarb and salt, then set over a medium-low heat, cover and leave the rhubarb to soften for about 10 minutes.

Lift off the lid and turn up the heat to boil off some of the excess liquid for 10 minutes or so.

Take off the heat, stir in the mustard and butter, then taste and add more sugar, if need be.

Transfer to a clean jar and set aside until ready to use (or store in the fridge for two to three days).

Run the celeriac through the julienne attachment on a food processor.

Alternatively, cut it into 1-2mm-thick slices, stack them on top of each other, then cut into batons of a similar thickness.

Put the celeriac batons in a medium-large bowl and toss with the lemon juice.

Sprinkle over the salt, then scrunch this into the celeriac with your hands to soften.

Turn on the grill to its highest setting and line a grill pan with aluminium foil.

Stir the shallot and vinegar in a jug, then mix in the creme fraiche, mayonnaise and sugar.

Season to taste with pepper, then pour over the celeriac mix and toss until evenly coated.

Stir through the chopped herbs and capers, if using, and set aside.

Season the mackerel fillets with salt, then arrange them skin side up in the grill pan and cook for five minutes, until the skin is heavily blistered and the flesh cooked through.

Serve with the remoulade and a spoonful of the rhubarb sauce.