Peel the tough outer leaves from the leeks and halve them lengthwise, starting ½ inch from their base to keep them intact. Rinse them with cold water to wash any sand from their layers. Bunch the leeks and tie together with butcher’s twine. Place in a large pot with the remaining carrots, the bay leaf, and thyme and cover with cold salted water. Bring to a simmer and cook for 20 minutes, or until the carrots are tender. Strain, and discard the thyme and bay leaf.