First peel the onions and cut them into slices for onion rings, then take the matjes fillet out of the packaging and dab a little with kitchen paper.
Layer the onion rings and the herring fillets alternately in a bowl with a lid. Then pour all the buttermilk over it and also the cup of cream. Carefully mix the two liquids with a fork. Finally, pour pepper from the mill over the whole liquid. Don`t stir anymore! Put the lid on and in the fridge until the next lunchtime.