Dice the apple, onion and cucumber really finely, mix with the dill and the juniper berries. Halve the matjes lengthways and cut into 1.5 cm pieces, mix with the other ingredients.
Mix lemon juice with salt (depending on the salt level of the matjes), sugar, pepper and cayenne pepper in a blender. Work in the sour cream and mix everything with the herring.
Let it steep for one night if possible, stir before serving.