2 pounds Yukon Gold potatoes (about 4 large), peeled, quartered lengthwise and cut into 1/2-inch slices
1 cup low-sodium chicken broth
1 cup water
Table salt
1 tablespoon sugar
2 tablespoons white wine vinegar
1 tablespoon Dijon mustard
1/4 cup vegetable oil
1 small red onion, chopped fine
6 cornichons (pickles), minced (about 2 tablespoons)
2 tablespoons minced fresh chives
Ground black pepper
1. Bring potatoes, broth, water, 1 teaspoon salt, sugar and 1 tablespoon vinegar to boil in 12-inch heavy-bottomed skillet over high heat. Reduce heat to medium-low, cover and cook until potatoes offer no resistance when pierced with paring knife, 15 to 17 minutes. Remove cover, increase heat to high (so cooking liquid will reduce), and cook 2 minutes.
2. Drain potatoes in colander ser over large bowl, reserving cooking liquid. Set drained potatoes aside. Pour off and discard all but 1/2 cup cooking liquid (if 1/2 cup liquid does not remain, add water to make 1/2 cup). Whisk remaining tablespoon vinegar, mustard, and oil into cooking liquid.
3. Add 1/2 cup cooked potatoes to bowl with cooking liquid mixture and mash with potato masher or fork until thick sauce forms (mixture will be slightly chunky). Add remaining potatoes, onion, cornichons, and chives, folding gently with rubber spatula to combine. Season to taste with salt and black pepper. Serve warm or at room temperature.