Heat the oil in a large saucepan over medium-high heat. Add the onion, garlic, thyme, cinnamon, allspice, cloves, salt, pepper and sugar and cook until the vegetables are soft and lightly browned, about 5 minutes. Add the sweet potatoes, stock or broth, milk and cream. Cook with bubbles just forming on the surface.
Remove from the heat and puree the soup until smooth. This can be done in the pot with a handheld stick (immersion) blender or in a blender or food processor. (If using a blender or food processor, strain the soup, puree the solids and add the hot liquid back slowly in small batches, pulsing between each addition.) Adjust the thickness by adding more stock, milk or cream. Add sherry, if desired, stirring to combine.
Serve in individual bowls drizzled with truffle oil and honey and garnish with chives, if desired.
Makes six 8-ounce servings.