Wash the matjes fillets in cold water, pat dry with kitchen paper, halve lengthways and cut into small pieces. Peel and quarter the apple, remove the core and cut into thin slices. Quarter the pickles lengthways and cut into small slices. Peel and quarter the red onion, cut into thin slices and pull apart in strips. Put all ingredients (matjes fillet pieces, apple slices, onion strips, pickled cucumber slices, 1 teaspoon of pickled green pepper, 1 tablespoon of lemon juice, 1 tablespoon of Sylt salad sauce and 1 cup of 150 g Greek yogurt) in a bowl, mix carefully and refrigerate until serving.