Pull out any pin bones from the mackerel using tweezers, cut the fillets into large diamonds and put them in a deep dish.
Cut the lemon half vertically through the middle, then slice it into thin semi-circles. Place these in between the mackerel diamonds, then scatter over the crushed juniper berries, salt, pepper and thyme.
Combine the water, sugar and lemon juice for the marinade in a saucepan and bring to the boil. Toss in the licorice sticks and star anise and simmer for 1 minute.
Pour the marinade over the mackerel diamonds while still hot. Cover with cling film and leave at room temperature until completely cold, then refrigerate for 2-3 hours depending on the size of the fish, a maximum of 2½ hours for small fillets, otherwise the consistency of the fish suffers.
Serve the mackerel straight from the dish, allowing everyone to season their own fish.