Slice warm potatoes about 1/4" thick. Place in a large bowl.
Pour onion, vinegar mixture over warm potatoes.
Let sit approximately 10 minutes so that the potatoes can absorb some of the liquid.
There will be a little liquid left. This is what makes this salad so nice and creamy.
Add mayonnaise and stir gently so not to break up the potatoes.
Chill for at least two hours.
Sharing with Weekend Potluck