Position a rack under the broiler and preheat the broiler to high. Arrange tomatillos, serrano chiles (stemmed and seeded), onion slices, and garlic in a shallow broiler-safe pan.
Broil the vegetables until the tomatillos are soft and slightly charred, about 12 minutes, turning the pan once halfway through.
Transfer the broiled tomatillos, chiles, onion, and garlic to a blender; add 1/4 cup roughly 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, reduce heat to a simmer, and cook until the sauce thickens enough to coat the back of a spoon, about 20 minutes.
Remove the sauce from the heat and whisk in 3/4 cup Mexican crema (or crème fraîche or heavy cream) and about 1 teaspoon kosher salt. Let cool slightly before assembling.
While the sauce simmers, preheat the oven to 400°F (200°C). Rub 1 tablespoon vegetable oil over the chicken thighs, season with kosher salt and freshly ground black pepper, and bake until internal temperature reaches 165°F (about 45 minutes for bone-in thighs).
Remove chicken from the oven and let cool slightly. Discard the skin, shred the meat with forks, and lightly season the shredded chicken with salt and pepper. Set aside.
Wrap the corn tortillas in moist paper towels and microwave on high for 1 minute, then an additional 30 seconds if needed, until soft and pliable.
Spray a 9"x13" baking dish with cooking spray. Dip each tortilla into the prepared verde sauce to coat, place in the dish, add a heaping tablespoon of shredded chicken in the center, roll up, and place seam-side down. Spoon a little sauce over the rolled tortilla to help seal. Repeat with remaining tortillas.
Pour 1 1/2 cups of the sauce evenly over the filled enchiladas, top 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. Serve immediately.