Chop the rhubarb into 2cm diagonal pieces. Peel the ginger and thinly slice it. Cut the slices in to thin matchsticks. Halve the lemon grass and bash it a couple of times with a blunt knife to release the flavour.
Put all the pickle ingredients into a pan except for the rhubarb. Bring to the boil then simmer for 5 mins.
Put the rhubarb into a large tub or non-metallic bowl and pour the hot pickling liquor over the top. Using a spoon, submerge the rhubarb. Leave to cool then pop it into the fridge, covered tightly.
Pickle for a few hours or up to 4 weeks. This will make approx. 1kg of pickled rhubarb and much more that you’ll need for this recipe. Fantastic with cheese, pork chops or atop a curry or stirfry to add zing and crunch.
To cook the mackerel: place a frying pan on the heat for a few mins then pour in 1 tbsp olive oil. Season the mackerel fillets all over, and then lay them skin-side down in the hot pan. Fry for 2-3 mins, flip over using a fish slice or spatula, and cook for a further 1 min.
Serve the mackerel with a spoonful of the pickled rhubarb along with the pickling juices and a handful of peppery watercress.