In a medium saucepan, melt the butter over medium-high heat until it begins to foam slightly. Add the chopped onion and potato cubes, tossing gently to coat them evenly in the melted butter. Sauté for about 3 minutes, stirring occasionally, until the onions turn translucent and the potatoes start to soften at the edges. Season with a pinch of kosher salt and freshly ground black pepper.

Pour in the vegetable broth, followed by the heavy cream and half and half. Stir to combine and bring the mixture to a gentle boil. Once boiling, reduce the heat to low and let the soup simmer for about 25 minutes, or until the potatoes are tender and easily pierced with a fork.

Stir in the spicy whole-grain Dijon mustard, mild yellow mustard, and sugar. Continue to cook for a minute to allow the flavors to meld. Using a hand mixer or immersion blender, puree the soup directly in the pot until smooth and creamy. If a chunkier texture is preferred, blend only part of the mixture.

Taste and adjust the seasoning with additional salt and pepper if needed. Remove from heat and ladle the soup into bowls. Garnish with a light dusting of mustard powder and a few sprigs of fresh parsley before serving. This hearty, tangy soup is best enjoyed warm, paired with crusty bread or a crisp side salad.