In a large heavy bottomed skillet add the sliced potatoes, broth, water, salt, sugar and 1 Tablespoon of the vinegar to a boil.

Reduce heat to simmer, cover and cook until potatoes are tender.

Remove the cover and increase the heat to high to reduce the liquid (approx. 2 minutes)

Drain the potatoes reserving ½ cup of cooking liquid

Add water to make ½ cup if necessary.

Whisk in the remaining vinegar, mustard and oil into the cooking liquid

Add about ½ cup of the cooked potatoes into the cooking liquid and mash.

Pour the thickened sauce over the potatoes and add in the onion, cornichons and chives

Season to taste with salt and pepper

Serve warm or at room temperature