Remove any bones from the matjes fillets, wash, pat dry and cut into 2 cm pieces. Peel the onions, cut into slices and blanch them in boiling salted water for about 3 minutes. Peel, quarter, core and slice apples. Also cut the cucumber into slices.
Mix the whipped cream and sour cream with the lemon juice, season with pepper, salt and sugar. Add apples, cucumber and matjes fillets and stir well. Let it steep in the refrigerator for 12 hours.
Cook the potatoes in enough salted water, peel them and cut into wide slices. Heat the oil in a pan, fry the potato slices on each side until golden brown. Salt.
Serve the crispy fried potatoes with the herring fillets.