To start the salsa, first peel and wash the tomatillos. Then place into a pot, along with the Serrano peppers, cover with cold water, and bring to a simmer.

In the meantime, peel and roughly chop the garlic. Then finely dice the onion and set aside, while you chop the cilantro.

Once the tomatillos. turn a golden yellow color and start to split, remove them from the water.

Next, place the garlic into the blender. Cut off any stems from the serrano peppers and add peppers to the blender. Once the tomatillos are cool enough to handle, remove any large cores. Do not remove these prior to cooking or they will become water-logged. Add the tomatillos and salt to the blender and pulse just a few times. If you still see big chunks of chilies, pulse a couple more times.

Pour into a bowl and add the onion and cilantro. Mix and taste for seasoning. Depending on the tartness of the tomatillos, you may want to add a squeeze of lime juice. At this point, this dish is called Salsa Verde. It can be stored in the refrigerator for at least a week and can even be frozen. 

To make this into Salsa Verde de Lupita, simply dice the avocado and gently stir in.

Note: If you would like a spicier salsa, just boil another serrano pepper and blend it with a little bit of water. Mix it into the salsa to taste. Alternatively, you can char the tomatillos, serrano peppers and onions, rather than boiling them, for a more smoky end flavor.