Pre eat oven to 200°C.
Wrap beetroot in tin foil and cook for 40 minutes until soft in the center.
While the beetroot cooks, make the marinade for the fish. Peel and crush garlic and grate ginger. Mix in a bowl with the honey and olive oil. To prevent the mackerel fillets from curling when cooking, make three scores across the skin of each, before covering with the marinade. (For a quicker alternative way to cook the mackerel, see below)
Refrigerate for at least 30 minutes.
When the beets are cooked, leave to cool slightly before running under the tap while peeling the skin (be careful of the natural pink dye). Cut the beetroot into small cubes.
For the pickling liquor, add the sugar, cider vinegar, bay leaves and 150ml water to a saucepan and bring to the boil. Reduce the heat and simmer for 2 minutes.
Add the balsamic vinegar and beetroot. Refrigerate to cool.
For the yoghurt sauce, toast cumin seeds in a dry frying pan until aromatic then pour into a bowl. Finely chop the coriander and add to the bowl along with the yoghurt, olive oil and a good pinch of salt and pepper. Give everything a good stir with a spoon.
Pop the grill on medium high. Remove the mackerel fillets· from the fridge and cook under the grill for 4 minutes on each side, basting with the leftover marinade as you turn. Serve hot or cold with the pickled beetroot and yoghurt sauce.
Serve hot or cold with the pickled beetroot and yoghurt sauce.