Husk, rinse, and pat the tomatillos dry, then place on a baking sheet. Roast them under a preheated broiler until blackened in spots, about seven minutes. Turn them over and continue to blacken, another seven minutes. Remove from the broiler and set aside to cool at room temperature.

Cut the onion into quarter-inch slices. Heat a cast-iron skillet over medium heat for five minutes. Add the garlic, onion slices, and chiles and roast, turning the from time to time, until softened slightly and blackened in spots, about six minutes. Turn off the heat, remove the vegetables from the skillet, and set aside to cool at room temperature. Once they are cool enough to handle, peel the garlic clove and discard the skin. Remove and discard the stems of the chiles. Set aside.

Place all of the ingredients except the cilantro in a blender, and pulse to combine. Stir in the cilantro before serving.