Mix sour milk with yogurt and mayo and add enough cucumber juice (from the jar) to make a creamy sauce. Season with pepper and sugar.
Cut the rinsed matie fillets diagonally into 1 cm wide strips. Cut the peeled onion into strips and the cucumber into slices. Remove the skin from the apple, cut it into quarters, remove the core and divide it into fine slices.
Mix everything with the sauce and cool for 6 hours with the lid closed. Season to taste and add a little salt if necessary.