Preheat your oven to 180°C. Take a large baking tray, add a dash of olive oil and place in the oven.
Dice the sweet potato, crush the garlic, quarter the beetroot, prep the thyme, chunk the mackerel.
When the tray is hot, place the sweet potato and crushed garlic on it, season and cook in the oven for 15 minutes, until soft.
Meanwhile, heat a dash of olive oil in a frying pan, then add the beetroot and thyme.
After four minutes, add the red wine vinegar and 100ml water, cover but keep an eye on it because you may need to add more water. Simmer until the beetroot is cooked (about 15 minutes).
When the beetroot is ready, allow to cool, then mix with the sweet potato and flaked mackerel. Season, finish with the flat leaf parsley, and serve with the horseradish.