Gently toss boiled potatoes, parsley, onion, hard boiled eggs, and bacon together.

In a saute pan whisk reserved bacon fat with flour until smooth.

Slowly whisk vinegar and water in.

Add sugar and salt.

Stir continuously until thickened.

Pour over potato mixture. You may only use 2/3 of the sauce or use it all depending on how saucy you like your potato salad. If I'm making this ahead of time and reheating, I reserve some of the sauce to add after warming. It tends to absorb all the sauce as it reheats and can become dry.