Marinate the onion: Dice the onion, add into a small bowl, mix with vinegar (the onion should be completely covered with vinegar). Leave for 15 minutes.
Cook the potatoes and eggs: Peel the potatoes and cut into 1-1.5 inch/3 cm cubes. Add to a pot with boiling salted water. Cook the potatoes for 6 minutes, then add the eggs and cook for 8 more minutes. Drain.
While the potatoes and eggs are cooking make the dressing: Strain the onion, reserve the vinegar and add it into a medium jar or small bowl. Add the rest of the ingredients for the sauce. Season with salt and pepper. Shake the jar thoroughly to mix the ingredients or whisk the ingredients in the bowl until combined.
Stir the potatoes with the dressing: Transfer cooked potatoes into a large serving bowl and immediately pour in the sauce (potatoes should be still warm, they will absorb the sauce), stir the potatoes with the sauce.
Chop the eggs, apples, cucumbers, and dill: Peel the eggs and apples and chop into small cubes. Chop the cucumbers. Chop the dill finely. Add them to the bowl.
Combine all salad ingredients. Season with salt and pepper, if necessary. If the salad is not sour enough, you can add more vinegar or lemon juice, if it's too sour for your liking, you can add some honey.
Enjoy!