To begin, rinse the matjes fillets under cold water to remove excess salt. Pat them dry with paper towels and set aside. This step ensures the flavour balance of the dish and removes any overpowering saltiness.

Prepare the pickling brine. In a saucepan, combine the white vinegar, water, sugar, mustard seeds, black peppercorns, and bay leaves. Heat the mixture over medium heat, stirring until the sugar dissolves. Allow the brine to cool completely before use.

Layer the matjes fillets, onion slices, and apple slices alternately in a glass or ceramic container. Ensure the fillets are laid flat to absorb the brine evenly.

Pour the cooled brine over the layered ingredients, ensuring they are fully submerged. Cover the container tightly and refrigerate for at least 24 hours to allow the flavours to meld. For best results, let the matjes marinate for up to 48 hours.

While the matjes marinate, prepare the accompaniments. Boil the potatoes in salted water until tender. Drain and let them cool slightly. For extra flavour, toss the potatoes in melted butter and sprinkle with chopped dill.

Prepare the sour cream sauce. In a small bowl, mix sour cream or crème fraîche with a pinch of salt, black pepper, and a handful of freshly chopped dill. This creamy component complements the tangy matjes beautifully.

When ready to serve, remove the matjes from the brine and arrange them on a platter. Discard the brine but retain the onions and apples for garnish. Present the fillets alongside the boiled potatoes, sour cream sauce, and pickled cucumbers.

Serve the matjes chilled with a sprinkle of fresh dill for garnish. For an authentic German touch, pair the dish with dark rye bread and a light pilsner beer. Encourage diners to create bites combining the fillets, potatoes, and sauce for a harmonious blend of flavours and textures.