Place all of the ingredients into a saucepan with the water.
Bring to a boil over high heat, then reduce heat to medium-low. Simmer until tomatillos are soft (10 to 15 minutes).
Remove the ingredients with a slotted spoon into a blender jar, leaving the water in the saucepan.
Pulse the cooked vegetables in the blender until all ingredients are finely chopped and mixed. Season with salt, to taste.
You can chill it or serve warm with tortilla chips and enjoy.
Notes
If you enjoy more heat, don't remove all of the serrano chili pepper seeds/membrane, as that is where the majority of the heat comes from.
Cooked, homemade tomatillo salsa can last up to a week in the fridge in an airtight container. If it's become moldy, it's gone bad. You can freeze the salsa for up to 2 months.