Remove husks from tomatillos, add to bowl and rinse under warm water. Roughly chop tomatillos into 1 inch pieces.
Peel and dice the onion, slice the serrano peppers and chop the cilantro.
In a sauté pan, heat olive oil on medium heat. Add the onions and cook until translucent. Then add the tomatillos, cilantro, peppers, salt and black pepper. Stir well and cook for about 5 minutes until tomatillos are tender. Remove from heat.
Add tomatillo mixtures to a blender and puree mixture until it's a soupy consistency. Squeeze juice of 1 lime and quickly blend. Taste to make sure sauce has enough salt for your taste and adjust.