Boil the potatoes in their skins in lightly salted water until tender. Allow the potatoes to cool until you can handle them. Peel the potatoes and slice them into 1/4 inch slices. Put the sliced potatoes in a large mixing bowl and set aside.
Add onions, beef broth, vinegar, salt, pepper, sugar, and mustard in a medium saucepan and bring to a boil. As soon as it boils, remove from heat and pour the mixture over the potatoes. Cover the bowl of potatoes and let sit for at least one hour.
After at least one hour, gently stir in the oil and season with salt and pepper to taste. If too much liquid remains, use a slotted spoon to serve. Serve garnished with fresh chopped chives. Serve at room temperature. Note: This potato salad is best the next day (remove from fridge at least 30 minutes before serving).
Read the "7 Tips" in this blog post to ensure success!