Preheat the oven the day before (electric cooker: 200 °C/circulating air: 175 °C/gas: see manufacturer). Wash beetroots thoroughly and dab dry. Wrap firmly in aluminium foil and place on a baking tray. Bake in a hot oven for about 1 1⁄2 hours until they are completely soft.
Take out, remove aluminium foil and peel the still hot tubers. Halve them and put them in a high container. Pour beer and vinegar over it. The beetroots should be completely covered. Marinate for about 24 hours.
The next day, pour the beetroot into a sieve, drain very well. Peel, wash and thinly slice the carrots. Pour into boiling salted water, bring to the boil and cook for about 1 minute. Drain, rinse with cold water and drain well.
For the sauce, mix soured milk, 3 g (approx. 1⁄2 tsp) salt and liquorice in a bowl. Cut beetroot into cubes (approx. 0.8 x 0.8 cm). Wash the apple, remove seeds, peel and cut into small cubes (approx. 0.5 x 0.5 cm).
Carefully mix the beetroot, carrots and apple with the seasoned soured milk and leave to soak for about 1 hour.
Cut the mackerel fillets with a sharp knife into 4 pieces each. Arrange each 1⁄4 salad and 2 pieces of mackerel. Rye bread tastes good with it.