Heat cast iron pan or griddle to high heat. Place tomatillos and garlic cloves in pan. Let the tomatillo’s skin burn and blacken on all sides. The garlic needs to be only lightly toasted on each side, about 1 -2 min.
Remove garlic from pan, peel, and set aside.
Once the tomatillos are soft and mostly black, remove them from pan and place in a medium bowl. Cover with plastic wrap and let sit for 5 min.
Bring 3 cups of water to a boil in a small sauce pan.
Remove the stem and seeds from the chiles. Place them in the pot of boiling water and simmer from 5 to 7 minutes or until chiles are soft.
Drain the chiles from the water.
Place the tomatillos in the blender with the garlic, onion, cilantro, and chile de arbol. Blend, season, and add water is necessary to thin out sauce.
As an alternative you can place the tomatillos under the broiler in your oven for 15 to 20 min. until blackened and soft all over.