Peel the potatoes, don't need to cut them into pieces. Add it to a pot with salt and water (water should be cold). All the potatoes should be covered with water.
Once the water is boiling, let it cook for 15-20 minutes (until fork tender)
Remove the potatoes from the pot and let them cool for a few minutes so you can handle them without burning yourself. Then place the potatoes into a cutting board and cut the potatoes into smaller pieces.
Place the potatoes in a big bowl.
In a small pot, add the onion, water, vinegar, salt, pepper, sugar, and mustard. Let the pot boil, then put the mixture on top of the potatoes. Let it sit for an hour. Stir (gently) the potatoes, add the olive oil, adjust pepper and salt as you like. Stir it well and place the potato in the fridge.
Serve the next day for better results.