Boil vinegar, wine, lemon rind, star anise, salt, and peppercorns in a pan.
Pour liquid into a tray and submerge mackerel fillets skin-side up to cool.
Melt 55g butter with sugar, add ginger and chopped rhubarb, and cook until puréed.
Stir in grenadine and rhubarb batons, cooking gently to keep batons whole.
Remove mackerel from liquid, pat dry, and fry skin-side up in oil for five minutes.
Baste mackerel continually with 30g of butter while frying.
Heat remaining butter until golden-brown, then stir in lemon juice, parsley, and fennel seeds.
Plate the rhubarb purée, top with mackerel, and spoon the sauce over.