Marinated mackerel with spiced aioli

Method

1. Remove any pin bones from the fish fillets with tweezers and put skin-side down and side-by-side in a deep dish. Scatter with the onion, dill, bay leaves and horseradish. Chill.
2. Meanwhile, combine 125ml water with the sugar, wine vinegar and spices in a saucepan, and stir over a low heat to dissolve the sugar. Bring to the boil, then immediately remove from the heat and leave to cool completely.
3. Pour the cold marinade over the mackerel, cover the dish with cling film and marinate in the fridge for at least 3 hours, but preferably overnight, before serving.
4. For the aïoli, put the saffron, garlic, egg yolks, potato (if using) and lemon juice into a blender or food processor. Blend until thick and smooth. With the motor running, slowly trickle in the oil until mixed. Season well.
5. Drain the mackerel fillets and place on plates with some of the marinated onion rings, garnished with sprigs of bay leaves and dill. Serve with the aïoli, a green salad and some slices of toasted focaccia or warm baguette.