In a 3- to 4-quart slow cooker, combine celery, onions, carrots, clam juice, chicken broth, thyme, salt, and pepper.
Cover and cook on low-heat setting for 4 1/2 to 5 hours or on high-heat setting for 2 to 2 1/2 hours.
If using low-heat setting, turn to high-heat setting. Combine in a small bowl the half -and-half and cornstarch and mix together, slowly add clams and mix together. Add sherry into vegetable mxiture (optional). Cover and cook for 30 minutes. Sprinkle each serving with crumbled bacon and, if desired, green onions. Enjoy!