Heat the oil in a large saucepan over medium heat. Add the leeks, celery, and garlic and cook, stirring often, until softened, about 5 minutes. Add the pumpkin cubes and canned puree, then the broth. Simmer until the pumpkin is tender, about 25 minutes. Stir in 1 3/4 teaspoons salt and 1/4 teaspoon pepper. Working in batches, ladle the soup into a blender and puree until smooth. Divide among individual bowls and top with the rosemary.