1. Place the chicken stock, sweet potato or squash, onion, celery, and bouillon in a 2 1/2-quart pot. Bring to a boil over high heat. Then reduce the heat to low and cover the pot. Simmer for 30 minutes, or until the vegetables are very tender.
2. Use a slotted spoon to transfer the vegetables from the pot to a blender or food processor. Add 1 cup of the hot broth, and process, with the lid slightly ajar, for 30 seconds, or until the mixture is smooth. Return the blended mixture to the pot.
3. Add the matzo balls to the soup, and simmer, covered, for 10 minutes.