Preheat your broiler, then place the tomatillos, jalapenos, and garlic on a baking sheet and broil for 5–7 minutes (just until the tomatillos are slightly charred).
Allow the ingredients to cool slightly, then place them in a blender or food processor, adding the cilantro, salt, and onion.
Blend until smooth.
Taste and adjust your seasonings if you need to.