Put the tomatillos whole in a small saucepan, barely cover with water, and bring to a simmer. lower the heat and continue cooking until soft but not falling apart, about 5 minutes depending on size. Drain, reserving the cooking water.
Using a molcajete: Mash and grind together the chiles, 1/2 cup cilantro, the garlic, onion, and salt. Add the tomatillos a few at a time, crushing and grinding well after each addition and adding a little of the cooking liquid until you have a textured sauce of medium consistency. You will probably need about 1/2 cup or even more of the cooking liquid.
Using a blender: Put 1/3 cup of the cooking water into the blender jar. Add the chiles, 1/2 cup cilantro, the garlic, and the onion and grind to a paste. Add the tomatillos a little at a time, blending with about 1/3 cup more of the cooking liquid until you have a textured sauce of medium consistency. Add salt to taste.
To serve: Sprinkle the top with the extra cilantro.