Soused Aromatic Mackerel

Thanks to Mitch Tonks for the recipe from his lovely book Fresh.
You can make the aromatic liquid up to the day before if you want to cut the cooking time down
Ingredients
2 Mackerel, filleted
2 pieces Star Anise
½ tsp Coriander seeds
½ tsp Mustard seeds
4 – 5 All Spice (or Juniper) berries
½ tsp Black peppercorns
6 Cloves
Pinch of Mace
2 Tiny dried chillies
2.5cm Root ginger, peeled and chopped
2 Bay leaves
150ml White wine vinegar
50ml Water
50ml Dry white wine
4 tbsp Sugar
3 cloves of Garlic, finely sliced
1 small Onion, finely sliced
Handful of fresh mint, chopped
Preheat the oven to 180°C.
Put all the dried spices and the ginger (but not the bay leaves) in a pestle and mortar. Bash them to break the seeds and berries open. Place in a small saucepan with the bay leaves, vinegar, wine, water and sugar. Bring to the boil and simmer for 10 minutes. Strain the liquid into a bowl and discard the spices. Add the garlic and onions to the saucepan, cover with the spiced liquid and simmer with a lid on for a further 10 minutes.
Place the fish flesh side down in an ovenproof (non-metallic) dish. Season with a little salt and pour over the liquid, including the onions and garlic. The fish will be just covered to halfway, if not add a little water. Cover the dish with a lid or tightly sealed tin foil and bake in the oven for 20 minutes. Sprinkle over the mint.
Serve with mash potatoes and steamed green beans.