Place the leeks, squash, bay leaf, salt, and broth in a large saucepan. Bring to a boil. Reduce heat and simmer gently until the squash is tender, about 12 minutes. Let cool for at least 10 minutes. Remove and discard the bay leaf. Puree the soup in batches and rewarm over medium-low heat.
Meanwhile, place the pumpkin seeds (if using) and rosemary on a cutting board and roughly chop. Heat the oil in a small skillet over medium heat. Add the seeds and rosemary and heat, stirring occasionally, until fragrant, 2 to 3 minutes. Ladle the soup into bowls and sprinkle with the seeds and rosemary.