Mix 1 cup water and cornstarch in a bowl until cornstarch dissolves.
Heat olive oil and garlic in a large pot over medium heat for 1 minute. Add carrots, celery, and salt; cook until tender, 5 to 10 minutes. Add chicken thighs; cook until no longer pink in the center and the juices run clear, about 7 minutes. Add remaining 7 cups water and chicken soup base.
Bring to a boil, coaxing anything stuck to the bottom of the pot off with a spatula. Boil for 20 minutes. Add cornstarch mixture, sage, and pepper; mix well and continue to boil for 5 minutes. Remove from heat and add lemon juice.