For the mackerel, put the vinegar, wine, lemon rind, star anise, salt and peppercorns in a pan and bring to the boil. Pour into a tray and lay the mackerel fillets in the liquid skin-side up. Leave to cool.
For the rhubarb, melt the butter with the sugar in a clean pan. Add the ginger and roughly chopped rhubarb. Cook gently until it becomes a purée, then break it up using a wooden spoon. Add the grenadine and rhubarb batons. Cook gently, keeping the batons whole.
Meanwhile, take the mackerel fillets out of the liquid and carefully dry. Heat the oil in a frying pan, add the mackerel, skin-side up, and cook for approximately five minutes. Meanwhile, add 30g/1oz butter and continually baste the mackerel with this.
In a clean pan make the sauce. Heat the butter until it starts to turn golden-brown. Remove from the heat and add the lemon juice, parsley and fennel seeds.
To serve, lay the rhubarb in the middle of serving plates. Carefully lay the mackerel on top. Spoon the sauce over and serve.