Position a rack under the broiler and heat the broiler to high. Arrange the tomatillos, serrano chiles, onion slices, and garlic in a shallow roasting pan.
Broil the vegetables until the tomatillos are soft and slightly charred, about 12 minutes, turning the pan halfway through.
Transfer the roasted tomatillos, chiles, onion, and garlic to a blender, add 1/4 cup chopped cilantro, and blend until smooth.
Heat 2 tablespoons vegetable oil in a medium saucepan over medium-high heat. Add the blended tomatillo mixture and cook, stirring, for 3 minutes.
Add 2 cups chicken broth to the saucepan and simmer until the sauce is reduced slightly and coats the back of a spoon, about 20 minutes.
Remove the sauce from heat and whisk in 3/4 cup Mexican crema (or crème fraîche or heavy cream) and 1 teaspoon kosher salt; set aside to cool slightly.
Meanwhile, preheat the oven to 400°F (200°C). Rub 1 tablespoon vegetable oil over the chicken thighs and season with kosher salt and freshly ground black pepper.
Bake the chicken thighs until cooked through (internal temperature 165°F), about 45 minutes for bone-in thighs. Remove from oven, let cool slightly, discard skin, and shred the meat with a fork. Lightly season the shredded chicken with salt and pepper.
Wrap the corn tortillas in a damp paper towel and microwave on high for 1 minute to soften; microwave an additional 30 seconds if needed so tortillas are pliable.
Spray a 9"x13" baking dish with cooking spray. Working one at a time, dip a tortilla into the verde sauce to coat it, place it in the dish, add a heaping tablespoon of shredded chicken in the center, roll the tortilla, and place seam-side down. Spoon a little sauce over the rolled enchilada to seal. Repeat with remaining tortillas.
Pour 1 1/2 cups of the sauce over the assembled enchiladas, sprinkle with 1 1/2 cups shredded cheese, and bake at 400°F until bubbly and lightly browned on top, about 25 minutes.
Garnish the baked enchiladas with chopped tomato, chopped cilantro, and crumbled Cotija cheese, then serve immediately.