Place whole, unpeeled potatoes in water and boil until they can easily be pierced with a fork.
In the meantime, dice your onion(s)
Once the potatoes are boiled, peel them while hot
Slice the potatoes a little more than or about 1/2 cm thick. They should not be too thin, or they will crumble.
Add the potatoes and onions to a bowl
Add a few tablespoons of vinegar, and equal parts water; a few pinches of salt and sugar. Start with less. Stir in. Do not add the oil just yet - only one or two hours before serving, as this should coat the potatoes, not be entirely soaked into them.
Continue to stir over time and TASTE - this will taste different in the beginning and over the next couple of hours. Add salt, sugar, vinegar or water when it is necessary - add pepper and parsley if you would lie.
Add the oil an hour or two before serving, a couple of tablespoons. Taste before serving, and add more if necessary.
Serve at room temperature.