Heat the olive oil in a large, heavy pot over medium heat. Add the onion, garlic, cilantro sprigs, cumin, and a large pinch of sea salt. Cook, stirring now and then, until softened but not browned. Add the chipotle and the sweet potatoes and stir to combine.
Add the vegetable stock 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 puree the soup in a powerful blender (make sure to keep the lid off and cover the top of the blender with a towel so the heat can escape. Start the blender on the lowest setting and very slowly increase the speed.) Garnish each bowl with the reserved cilantro and some olive oil.