Combine onions, 1 cup chicken broth, minced garlic, and next 3 ingredients in a Dutch oven over medium heat; cook, stirring occasionally, 25 to 30 minutes or until most of the liquid evaporates. Increase heat to medium high, and cook, stirring often to loosen particles from bottom of Dutch oven, 15 to 20 minutes or until onions are caramel colored.
Stir in beef broth, tomatoes and green chiles, chipotle pepper, and remaining chicken broth. Bring to a boil over high heat; reduce heat, and cook, stirring often, 15 to 20 minutes. Ladle soup into 6 bowls; sprinkle evenly with cheese, Tortilla Strips, and chopped cilantro.