Soup: Heat a heavy saute pan over medium-high heat. Add the tomatoes, peeled onion, and unpeeled garlic cloves. Roast, turning frequently, until the garlic is soft, about 10 minutes. Remove the garlic when soft, but slightly blackened. Peel the garlic and set aside. Add the chile and continue to cook for another 2 to 4 minutes. Remove the onion and set aside. Continue to cook the tomatoes until charred and blackened, another 5 minutes. Put the garlic, chile, onion, tomatoes, tortillas, and 2 cups chicken broth in a blender. Blend until smooth. Pour into a large saucepan and add the remaining 4 cups chicken broth. Bring the mixture to a boil, reduce the heat and simmer for 20 minutes. Season with salt and pepper, to taste.

Tortilla strips: In a medium, heavy skillet, pour enough oil to reach a depth of 1/2 an inch. Heat over medium-high heat until very hot, but not smoking. Add the tortilla strips in batches and fry until golden and crisp, about 2 minutes. Remove with a slotted spoon and drain on paper towels. Set aside.

To serve, ladle the soup into 6 individual serving bowls. Serve whole chipotle chiles, Mexican sour cream, avocados, cheese, and tortilla strips on the side.