Heat oil in a large pot over medium heat. Cook garlic and leek until soft and translucent, about 4 minutes. Add apple, celery, and ginger, and cook for 3 minutes. Add sweet potatoes and stock, and simmer until sweet potatoes are tender, about 15 minutes.
Add chipotle chiles. Puree until smooth. Season with salt and pepper. Sprinkle with pepitas before serving.