In a large saucepan, combine first 4 ingredients and 5-½ cups broth. Bring to a boil; cook 3 minutes.
In a small bowl, mix cornstarch and remaining broth until smooth; gradually stir into soup. Bring to a boil; cook and stir until thickened, 1-2 minutes. Remove from heat; stir in lemon juice and cilantro.
