Put 110ml/4fl oz water in a pan with the white wine vinegar, sugar and salt, and heat until the salt and sugar have dissolved.
Pour over the turnip and set aside to pickle for two hours, then drain off the pickling liquid.
Bring a pan of salted water to the boil, add the parsley and blanch then refresh in iced water.
Drain, squeeze dry then roughly chop and set aside.
Heat a frying pan until hot, add the butter and onion and cook for a few minutes until just softened.
Add the watercress and stock and bring to the boil then add the chopped blanched parsley.
Ladle in batches into a food processor then blend to a fine purée and pass through a fine sieve then set aside.
Bring a pan of salted water to the boil, add the asparagus and blanch for 2-3 minutes until just tender.
Drain and season the asparagus with a little olive oil, salt and black pepper.
Preheat the grill to high.
Place the mackerel onto a grill tray and season with salt and black pepper.
Grill for two minutes on each side.
To serve, place a spoonful of parsley purée onto the centre of each serving plate and smear across the centre of the plate. Place the mackerel on top and scatter the asparagus and turnips around the plate. Finish with a scattering of baby watercress and drizzle of oil.