Tacos with Chicken in Poblano Chile Sauce

Char the chiles directly on the flames of a gas stovetop with the heat at medium-high (you can do this directly on electric burners as well), turning them with tongs as they blacken until they are blackened all over, 6 to 8 minutes. Transfer the chiles to a bowl and let stand, covered with a plate or plastic wrap, to loosen the skins, at least 15 minutes. Peel the chiles, then slit them open and remove and discard the seeds, seed pod, and veins. Coarsely chop the chiles. Cook the onion and garlic in the oil in a large, heavy skillet over medium heat until softened, about 3 minutes. Add the poblanos and the salt and cook, stirring, for 3 minutes. Transfer the mixture to a blender, add the chicken stock or broth and blend until smooth.