Position a rack under your broiler, and turn on high. Arrange the tomatillos, chiles, onion slices, and garlic in a shallow baking/roasting pan. Broil, until the tomatillos are soft and slightly charred, about 12 minutes, turning the pan halfway through.
Transfer vegetables to a blender and add the cilantro. Blend until smooth.
Heat the 2 tablespoons of oil in a medium saucepan over medium-high heat. Add the tomatillo mixture and cook, stirring, for 3 minutes.
Add 2 cups of the broth and simmer until the sauce is thick enough to coat the back of a spoon, about 20 minutes.
Remove from the heat and whisk in the Mexican crema and Kosher salt. Let cool slightly before assembling the enchiladas.
While the sauce is simmering, preheat the oven to 400°F. Rub the vegetable oil over the chicken thighs, and then sprinkle all over with Kosher salt and fresh ground black pepper. Bake until cooked through with an internal temperature of 165°F. (Approx. 45 minutes for bone-in, 30 minutes for boneless).
Remove the thighs and let cool slightly. Remove the skin and discard it. Shred the meat with a fork. Lightly salt and pepper the shredded chicken. Set aside.
Place moist paper towels around the corn tortillas and place in the microwave on high for 1 minute. Microwave for another 30 seconds, if necessary.
Spray a 9"x13" baking dish with cooking spray. One by one, dip the tortilla into the prepared sauce, just enough to coat the tortilla. Lay the sauced tortilla into the dish. Fill the center of the tortilla with a heaping tablespoon of the cooked chicken, and then roll in the side of the tortilla, sealing them shut at the top. Repeat with remaining tortillas.
Pour 1½ cups of the sauce over the tortillas. Top with the cheese and bake until bubbly and lightly browned on top, about 25 minutes.
Garnish with chopped tomatoes, cilantro, and crumble Cotija cheese, if desired. Serve at once!
