I got this recipe from a friend who grew up in an old-fashioned German family from Illinois like my grandmother. It's just as good cold as hot.
Also, note that the recipe calls for two teaspoons of salt, not two Tablespoons like I read it one horrible morning that I made it for a church potluck lunch. It was a good thing I tasted it before it packed it up for church. And it was a good thing I had enough time to run by the store and buy an emergency-back-up-salad. Also, in God's gracious mercy someone else had also made German Potato Salad that day so it all worked out.
5 large potatoes
4 slices thick-sliced bacon
1 onion, diced
2 teaspoons salt
1/2 teaspoon celery seed
1/3 teaspoon dried (powdered) mustard
boil the potatoes. Set aside and cool a bit
cut the bacon into pieces and cook in an iron skillet-- take out of skillet and set aside
pour out all but 2 Tablespoons of the bacon fat
add the onion and cook until soft
add 1 Tablespoon flour to make a roux
when roux is cooked and smooth (as smooth as it can be with onion pieces in it)
add 1/2 cup cider vinegar
1/4 cup water
1/4 cup sugar
cook until thick gravy-like consistency
peel and slice the potatoes
add to the roux
add the reserved bacon pieces
You can eat this either hot or cold.