Place the leeks, squash, bay leaf, broth, ¾ teaspoon salt, and ¼ teaspoon pepper in a large saucepan or Dutch oven and bring to a boil.
Reduce heat and simmer, stirring occasionally, until the squash is tender, 10 to 12 minutes.
Remove and discard the bay leaf. Working in batches, puree the soup in a blender until smooth (or use a handheld immersion blender).
Meanwhile, heat the oil in a skillet over medium heat. Add the rosemary and seeds, if desired, and heat, stirring occasionally, until fragrant, 2 to 3 minutes.
Divide the soup among bowls and top with the rosemary mixture. Serve with the bread, if desired.