Cook the potatoes: Place unpeeled potatoes in a large pot and cover with cold water. Bring to a boil and cook until tender, about 20–25 minutes. Drain and let them cool just enough to handle.

Peel and slice: Peel the warm potatoes and slice them into thin rounds. Place in a large mixing bowl.

Sauté the onion: In a small saucepan, heat 1 tablespoon of oil over medium heat. Add diced onion and cook until translucent, about 3–4 minutes.

Make the dressing: Add the broth, vinegar, mustard, and sugar to the pan with the onions. Stir and bring to a gentle simmer. Season with salt and pepper.

Combine: Pour the warm dressing over the sliced potatoes. Gently stir to combine, being careful not to break the potatoes too much.

Rest: Let the salad sit for 20–30 minutes so the potatoes can absorb the dressing.

Finish and serve: Drizzle the remaining tablespoon of oil over the salad, garnish with chopped parsley if desired, and serve warm or at room temperature.