Austrian Potato Salad

Name: Austrian Potato Salad
Ingredients: 2 pounds (900 g) waxy potatoes, all about the same size, rather small to medium ones; 1 medium yellow onion, diced finely (3.5 oz / 100g); 2 tablespoons (28 g) unsalted butter; 1/3 cup (80 ml) white wine vinegar or mellow apple vinegar; 3/4 cup 1 tablespoon (200 ml) vegetable broth; 3/4 tablespoon tarragon mustard (substitute with savory-hot mustard like spicy brown if you can not find); 1/3 cup (80 ml) sunflower oil (or any other neutral-tasting oil); 1 teaspoon (8 g) fine sea salt or table salt; Chopped, flat parsley for garnish