Heat the broiler and adjust the shelf to its highest position. Spread the tomatillos, garlic and chiles on a baking sheet, slide under the hot broiler and roast until the tomatillos are soft and lightly blackened in spots (about 5 minutes per side—the garlic and chiles should be done in about the same amount of time). Cool to room temperature, scoop into a blender jar (including any juices from the tomatillos), add the sorrel and pulse until you have a puree that’s nearly smooth. Add a little water (usually about 2 tablespoons) to give the salsa an easily spoonable consistency. Taste and season with a little salt if it needs it (believe it or not, sometimes it doesn’t need any).
Pat the chicken dry with a paper towel and season with a generous sprinkling of salt.
In a very large (12-inch) skillet set over medium-high, heat enough oil to coat the bottom. Lay in the chicken in a single layer and strew the onion slices into the open spots.
As the chicken sears, nestle in the onion slices, turning them as they brown. When the meat is richly browned on one side, turn it over and brown the other side, about five minutes more.
Pour the sorrel-tomatillo salsa into the pan, spoon the mixture into warm corn tortillas and top with queso fresco.