Pre heat broiler on high.

Place tomatillos, onions, and the Serrano pepper on a baking sheet lined with aluminum foil. Place the pan directly under the broiler and broil for 5 minutes, until black spots start to appear. Remove the pan from the oven, and using tongs quickly turn over the tomatillos and the pepper. Place back in the broiler and broil for an additional 5 minutes.

Remove the pan from the oven, place all ingredients in a food processor, and blend until mostly smooth.