Heat the oil in a late, heavy pot over medium het. Add the onion, garlic, cilantro sprigs, cumin, and a heavy pinch of salt and cook, stirring now and then, until softened but not browned, 10 minutes. Add the chipotle and the sweet potatoes and stir o combine. Add the vegetable stock to the pot and turn up the heat. Once the soup comes to a boil, lower the heat and simmer until the sweet potatoes are very soft, about 30 minutes. Remove and discard the cilantro. Carefully purée the soup in a blender. Garnish each bowl with a few of the reserved cilantro leaves.