Boil the potatoes in a large pot covered with an inch of water over high heat until tender, about 20 minutes.

Meanwhile, cook the bacon in a skillet over medium heat until crispy. Remove the bacon and set aside, leaving the fat in the pan.

Add the onion to the pan and sauté until translucent, about 3-4 minutes.

Add beef stock to the pan and bring it to a simmer.

Turn down the heat and add vinegar, mustard, oil, sugar, salt, and pepper to the pan.

Let the potatoes cool slightly, then peel and cut them into ¼-inch slices or cubes.

Pour the hot dressing over the potatoes and mix gently, folding in the bacon pieces.

Let the salad sit at room temperature for at least 20 minutes before serving.