Heat the butter in a stock pot over medium heat. Add leeks and onions and cook until translucent. Add chicken stock and simmer for 20 minutes. Add sweet potatoes and simmer for 20 minutes or until potatoes are soft. Add the kosher salt, pepper, ginger, nutmeg, cloves, and brown sugar.
Cool soup slightly. Purée the soup in small batches in a blender until smooth. Stir in heavy cream. Return the soup to pot and bring soup to just under a simmer before serving.
For the garnish: Heat canola oil in a small heavy-bottomed pot over medium-high heat until the temperature on a candy thermometer reads 350°F. Carefully fry the leeks until light golden brown. Immediately remove from the oil and place on paper towels to soak up excess grease. Do not push down on leeks. Garnish each bowl with the fried leeks.