Melt butter in a large heavy pot over medium heat. Add onions and sauté until soft and lightly colored, 7 to 9 minutes, stirring occasionally. Add garlic, then curry powder, if using, and continue cooking for 1 minute longer, stirring often. Add cauliflower and potato, 4 cups of the stock, Silver Palate & Rough Mustard, cardamom and salt, then stir. Bring mixture to a slow boil. Cover and continue to cook until cauliflower is very tender, 12 to 14 minutes. Transfer to a food processor and purée until almost smooth. Return soup to pot, adding additional stock if needed, and heat until hot. Ladle into bowls, add a dollop of yogurt, if desired, and a few chives.