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