Heat the olive oil in a skillet over medium heat. Add the onions, jalapeño, and garlic, and cook until soft. Stir in the seasonings, and cook for 1-2 minutes until fragrant. Transfer mixture to the slow cooker.
Add the sweet potatoes, and stock to the slow cooker pot, and turn on to low. Cover and cook for about 4 hours, until potatoes are tender. Transfer the soup to a blender and puree until smooth. Add back to the pot and stir in the sour cream. Serve the soup garnished with the cheese and additional jalapeños, if desired.