Unlike the more traditional red salsa which is served with fresh and uncooked ingredients, this particular recipe is made from things that are cooked ahead of time before blending into a smooth sauce, one of these ingredients being the tomatillo itself (sort of like a green tomato).
Place the tomatillos, onion, garlic, and chili pepper into a saucepan with the water.
Add the cilantro, oregano and cumin to the pot.
Bring to a boil over high heat, then reduce heat to medium-low. Simmer until tomatillos are soft (10 to 15 minutes). Remove tomatillos, onions, etc. with a slotted spoon into a blender jar.
Pulse the cooked tomatillos, onions, garlic, cilantro, and chili pepper in a 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.