If you use precooked beetroots, chop them into squares. Mix half of them with the lemon juice and sour cream and blend them with a hand blender. If you use pickled beetroots, strain them through a colander. Mix half of them with the sour cream and blend them (leave off the lemon juice as they are already a bit sour due to the vinegar we used for pickling).
Cut the apple into quarters and remove its core. Chop it into small cubes and mix with the remaining beetroots and the blended mass.
Cut off the tails of the herring filets. Dab the filets dry and also chop them into small cubes. Peel the onion and finely dice it. Wash and dry the dill and chop its leaves.
Mix the curd with the beetroots and the onion to a homogenous mass. Fold in the herring and dill and season it all with salt and pepper.
You can make this spread in advance and keep it in the fridge where it can be stored for up to two days.