Combine onion, chicken broth, tomatoes, and taco seasoning in a large sauce pan; bring to a boil over medium heat. Stir in chicken; reduce heat to low, cover, and simmer until chicken is no longer pink, 4 to 6 minutes. Mix cornstarch and water in a small bowl until smooth; gradually stir into soup.
Bring soup to a boil over medium-high heat; cook, stirring occasionally, until thickened, about 1 minute. Top with Mexican cheese and cilantro.