For salsa:1. Heat the oven to 400 degrees. Put the tomatillos on a baking sheet and roast until the skins are lightly browned and blistered, about 20 minutes. Remove the tomatillos; when they're cool enough to handle, chop them finely, along with the chiles, saving their juices. 2. While the tomatillos are roasting, put the oil in a large deep skillet over medium heat. Add the onions and garlic and cook, stirring occasionally, until they are very soft and lightly browned, about 10 minutes. Add the tomatillos, chiles, oregano, stock, and a large pinch of salt and pepper; stir and bring to a low simmer. Cook, stirring occasionally, until the mixture thickens slightly, 10 to 15 minutes.3. Stir in the cilantro and lime juice, taste, and adjust the seasoning. Use an immersion blender to purée the finished sauce in the pan. Or cool the mixture slightly, pour into a blender or food processor, and purée carefully. Serve at room temperature or refrigerate for up to 2 days.For enchiladas:1. Prepare the salsa if you haven't already. (You may cool and refrigerate the salsa for up to 2 days; reheat gently before proceeding.)2. Heat the oven to 350 degrees. Spoon a thin layer of the salsa into the bottom of a 9 × 12-inch baking dish. Put about 1/2 inch of the oil in a large, deep skillet over medium-high heat. When hot but not smoking, cook the tortillas, one at a time, until softened and pliable, about 10 seconds. Add more oil to the pan as needed. Drain on paper towels.3. First, pull the chicken meat from the bone and break it into bite-sized pieces. Sprinkle a heaping tablespoon of shredded chicken in the center of each tortilla, roll tightly, and put the enchiladas in the prepared dish, seam side down. The rolls should be packed in snugly against one another. Cover the top with some more sauce and bake for 25 minutes. When the enchiladas come out of the oven, sprinkle them with the queso fresco, onion, and cilantro. Serve with lime wedges on the side and pass the remaining salsa at the table.