Pre-heat broiler to 500°F and place oven rack close to the top of the oven.
Place tomatillos, chili peppers (jalapeños and serranos) and onion on a pan and place under the broiler. Broil until the vegetables blacken a bit, then flip and blacken the other side. (This will only take a couple of minutes.)
Remove from the oven and let cool for 5 minutes. Place all the ingredients in a blender (or food processor) and blend until smooth.
Serve with chips or on a delicious taco.
