Scrub the potatoes and place them whole and unpeeled in a large pot. Cover with cold salted water, bring to a boil, then reduce the heat and simmer for 15–20 minutes, until a knife pierces them easily without the potatoes falling apart.

Drain the potatoes and let them cool until they are easy to handle. Peel if desired, then slice them about 1/4 inch (6 mm) thick while they are still warm.

Cook the bacon in a large skillet over medium heat for about 6–8 minutes, until crisp. Transfer it to a plate and leave about 2 tablespoons of bacon fat in the skillet.

Add the chopped onion to the bacon fat and cook for 3–5 minutes, until softened. Whisk in the vinegar, water or broth, sugar, mustard, salt, and pepper.

If using flour, whisk it first with a tablespoon of water to make a smooth slurry, then add it to the skillet. Bring the dressing to a gentle simmer for 2–3 minutes without boiling it aggressively.

Add the sliced potatoes and most of the bacon to the skillet. Fold gently until the potatoes are coated and heated through, about 2–3 minutes.

Taste the salad and adjust the vinegar, sugar, salt, or pepper as needed. Garnish with the remaining bacon and chopped parsley or chives, then serve warm or at room temperature.