Peel and wash the tomatillos. Place them into a pot, along with the serrano peppers. Cover with cold water and bring to a simmer. Peel and roughly chop the garlic and cilantro and finely dice the onion.
Cook the tomatillos until they turn a golden yellow color. You may have to push them down with a strainer to keep them under the water so they obtain even coloring. Once the tomatillos start to split, remove everything from the water.
Place the garlic into the blender. Cut off any stems from the serrano peppers and add them to the blender. Once the tomatillos are cool enough to handle, remove any large cores. Don’t remove these prior to cooking or they’ll 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 is salsa de verde. It can be stored in the refrigerator for at least a week and can even be frozen.
To make this into salsa de Lupita, simply dice the avocado and gently stir in.