Open the can of tomatillos and drain and rinse them in a strainer or, if using fresh tomatillos, remove the husks from the tomatillos and rinse them well under cold water to remove the sticky residue. Roughly chop the tomatillos into halves or quarters. Add the tomatillos to your blender.
Add your de-stemmed and (opt.) deseeded serrano pepper(s) to the blender along with the white onion, garlic, cilantro (stems removed if serving a chunkier salsa), and the salt.
For a smooth sauce, blend the sauce on low until the ingredients begin to come together, then blend on high for 20 – 30 seconds to fully pulverize the ingredients. For a chunky salsa, pulse the ingredients until the desired consistency is achieved. You may need to scrape down the sides halfway through.
Serve immediately. Best if used within one hour of blending.