Start cooking potatoes in unsalted cold water. Bring to boil and continue cooking potatoes in simmering water until they soften (expect at least 30-45 minutes). It's ok if their skin splits during it. Once cook all the way through (test with knife or toothpick), drain them and let them cool down.
Remove skin from potatoes. Once completely cold (at least 2 hours after cooking) slice them into about 5 mm (1/4") wide slices.
Split onion into halves and slice each halve as thinly as you can. Mix with potatoes.
Add salt, black pepper, and about one third of oil and vinegar to start mixing. Alternate adding a tablespoon of oil/vinegar at a time while mixing and taste all the way. Stop once salad is of desired taste. Quantities above are just for guidance. The final amount will depend on the exact oil and vinegard properties.
|1 kg||Yellow potatoes||8-9|
|1½||Large yellow onion|
|15 g||Salt||1½ tsp|
|0.5 g||Black pepper||¼ tsp|
|~180 ml||Oil||12 tbsp|
|~200 ml||White vinegard||13 tbsp|