Potato Salad (Kartoffelsalat)

From My Berlin Kitchen. The salad is best if dressed a few  hours before serving but it's also delicious eaten right away when it's still slightly warm.

Makes 4 servings

2 pounds Yukon Gold or other waxy potatoes
3 1/2 tablespoons unsalted butter
2 yellow onions, finely chopped
1/3 cup white wine vinegar
2/3 cup beef or chicken broth
1 tablespoon Dijon mustard
3 tablespoons olive or sunflower oil
1 teaspoon salt
Freshly ground black pepper
2/3 cup loosely packed flat-leaf parsley, minced

1. Wash the potatoes and put them in a pot with cold salted water just to cover. Bring to a boil with the lid on; then reduce the heat to medium and cook the potatoes until they are just tender. Depending on their size, this should take between 20 and 30 minutes. Drain the potatoes and let them cool for at least an hour or two and up to overnight.

2. Peel the potatoes and cut them into very thin slices, about 1/8-inch thick. Put the potato slices into a serving bowl.

3. Melt the butter in a 10-inch saute pan and add the onions. Mix well and cook for 3 minutes over medium heat. Pour in the vinegar and the broth. Add the mustard and stir well. Let the mixture simmer over low heat for an additional 3 minutes.

4. Carefully whisk in the oil, and then pour the hot marinade over the sliced potatoes and mix well. The potatoes will take a few minutes to absorb all the dressing. Add the salt and as much freshly ground pepper as you’d like.

5. Just before serving, add the minced parsley to the bowl and mix well. Taste for seasoning and serve.