Remove the husks from the tomatillos and rinse thoroughly to remove the sticky coating.
Add the tomatillos and serrano peppers to a medium saucepan and cover with water.
Bring to a gentle boil, reduce the heat, and simmer for 5 to 7 minutes, until softened and olive green.
Reserve about ½ cup of cooking water, then drain the tomatillos and peppers.
Transfer the tomatillos and serranos to a blender. Add the onion, garlic, cilantro, and salt.
Blend until mostly smooth but still slightly textured.
If needed, add reserved cooking water one tablespoon at a time to reach your desired consistency.
Taste and adjust salt as needed.
Let cool before serving with tacos, carnitas, eggs, quesadillas, enchiladas, or tortilla chips.