Wash the potatoes and cook in boiling salted water for about 20 minutes. In the meantime peel onions and cut them into small cubes. Put vinegar in a bowl and season with salt and pepper.
Add oil. Drain the potatoes and let them cool down a little. Cut the matie filets into small pieces. Dice cucumbers finely. Wash and quarter the apple and cut into fine slices. Sprinkle with some lemon juice.
Peel the potatoes, cut into not too thin slices and halve them. Pour the vinegar-oil marinade over them. Cut bacon into strips and fry briefly in a pan without fat. First pour the breakfast bacon with the fat over the potato salad and mix lightly.
Then fold in the matjes, cucumber and apple slices.