Add the tomatillos and garlic; roast over medium-high heat, tossing them until they began to char on all sides.
Lower the heat, place the chiles in the skillet, and toast them briefly until they begin to blister and change color. Turn often so that nothing burns.
Remove the tomatillos, garlic, and chiles; let them cool for a few minutes.
Peel the garlic cloves.
In a molcajete, food processor, or blender, briefly blend the tomatillos and garlic leaving some texture.
Break the chile into several parts and add, seeds and all, to the tomatillo mixture.
Blend again and add salt to taste. If it is too thick, a small amount of water may be needed.
Serve in the molcajete or a small bowl, stirring in the onion at the last minute.