Boil the potatoes with their skin on, peel them and cut them into 3-5 mm thick slices. Cut the onion into cubes and mix the whole thing with salt and pepper. If desired, vinegar can be added, my grandmother always took cucumber water from pickles and also cut some small cubes into it.
Then just mix in the mayonnaise, especially delicious homemade - the recipe is in the database.
Hard-boiled eggs or pieces of cheese can be added if desired.