By eHow Food & Drink Editor
Place the tomatoes, garlic, chiles and onion on a preheated cast-iron griddle or large skillet over medium heat. Cook until the tomatoes are soft and blackened in spots, the garlic skins are dark in spots, the serrano skins are blistered and blackened, and the onion is soft and dark brown in spots.
Pull each ingredient off of the griddle or pan when it looks and feels done. They won't all get done at once.
Allow garlic to cool slightly, then peel. Stem the serranos - remove the seeds if you like a milder sauce.
Peel the tomatoes but leave a little of the blackened parts. Place the tomatoes, serranos, garlic and onion in a blender or food processor.
eHow Food & Drink Editor