Heat the oil in a medium-size pot over medium-low heat. Add the celery and onion and cook, stirring frequently, until tender, about 10 minutes. Lower the heat if the vegetables begin to brown.
Add the chicken broth, sweet potato, and thyme, and stir to combine. Raise the heat to high and bring to a boil. Lower the heat and cook at a low boil, stirring occasionally, until the sweet potatoes are tender, 12 to 15 minutes.
Stir in the chicken and heat through.
Notes
If you follow a vegetarian diet, switch from chicken broth to vegetable broth, and leave the chicken out. For more protein, add 1 to 2 cups of chickpeas. Each serving has 160% vitamin A.