Salsa Verde

Remove the papery husks from the tomatillos and rinse to remove the sticky residue.
Quarter the onion.
Add all of the ingredients except the cilantro and salt to a large pot and just cover with water.
Bring the water and ingredients to a boil and then simmer for 10 minutes.
Blend the cooked ingredients and the cilantro with the cooking water until smooth. (About 30 seconds)
Heat 2 tablespoons of cooking oil in the pot.
Pour the blended salsa back into the pot with the hot cooking oil.
Reduce the heat and simmer for 20 minutes.
Add the salt and adjust if necessary.

Preparation

Too runny? Simmer until it has reduced to the desired consistency.
Too thick? Add water 2 tablespoons at a time until the desired consistency is reached.
Too hot or acidic? Add only one of the following to reduce heat: 1/2 teaspoon baking soda, 1 to 2 teaspoons sugar, 1 to 2 teaspoons agave syrup.
Like it hot? You do not have to seed the serrano chiles. You can seed them if you want to reduce the heat of the salsa.

Substitutions

You can substitute jalapeño chiles for the serranos but the flavor won't be as bright but it will still be tasty.

Storage

The salsa will keep in the refrigerator for 3 days.
This salsa freezes very well. Make a double batch so that you always have some on hand.