Heat the oven to 220C (200C fan)/425F/gas 7.
Bring a large pan of salted water to the boil. Add the potatoes and cook for 8 minutes. Once cooked, drain and allow to steam dry for 15 minutes.
Tip the potatoes back into the pan, put the lid on and shake the pan gently to ruffle the edges of the potatoes, but not so hard they break up.
While the potatoes dry, boil the eggs for 6 ½ minutes. Once cooked, plunge into a large bowl of ice water to cool down.
Peel off the shells and halve the eggs.
Take two baking trays. On one tray toss the potatoes, 2 tbsp of olive oil and some salt together. On the other, toss the beetroot, remaining olive oil, vinegar and sugar. Roast both for 30 minutes or until the potatoes are golden.
Meanwhile mix together the creme fraiche, horseradish, 1 tbsp of the chopped dill and a pinch of salt. Chill until needed.
Toss the potatoes and beetroot with chunks of mackerel and the remaining chopped dill.
Divide between two bowls and top with the halved egg, horseradish creme fraiche, and more dill.
Season with salt and pepper, then a squeeze of lemon.
