Place tomatillos, onion, and garlic cloves on a sheet tray lined with parchment paper.
Broil in oven for 10 min. The tomatillos will begin to blacken and soften. After the first 10 min. remove the garlic and set aside.
Flip the tomatillos and the onion over. Put back in oven. Broil for 10 more minutes.
Boil water in a small sauce pot. Take the stems off of the chiles and remove the seeds. Add chiles to the boiling water and turn down to a simmer. Let simmer for 15 min. Drain and set aside.
If using a blender, place all the ingredients in blender until you have reached the desired consistency.
If using a molcajete, start off by peeling the garlic and grinding it to a paste with the salt. Add the onion and continue to grind to a paste. Add the chiles and continue to grind. By now you should have a thick red paste.
Add the tomatillos one at a time and keep on grindin’ until the skins dissolve. At this point if the salsa is too thick you can add a little bit if water to loosen it up. Season with salt and pepper.
If you don't want to use the oven you can blacken the tomatillos on a comal or griddle.