1. In a large pot, heat the butter until melted. Add the onion, celery, and garlic and sauté over medium heat until tender and translucent, about 6 minutes.
2. Add the chopped potatoes and toss to coat with the onion mixture. Add the cashews, mustard, salt, pepper, and stock. Bring to a boil and lower the heat to a simmer. Cook covered until the potatoes are tender, about 20 minutes.
3. Purée soup with an immersion blender or in batches in a blender. Taste for seasoning. Serve immediately plain or with suggested toppings added on top or stirred in.
Bake bacon on a parchment-lined baking sheet in a 375°F oven for about 20 to 25 minutes, turning after 10 minutes. You can cook it longer or at a higher temp if you like it crispier.
You can also add a diced carrot to give the soup a more golden color.
*Peeling takes away some of the nutrients and fiber, but creates a smoother soup.