Preheat the oven to 375°F (190°C).
Season the chicken thighs with salt and pepper. Heat the olive oil in a cast iron skillet over medium heat. Add the chicken thighs and cook for about 7-10 minutes on each side until they're cooked through and golden brown. Shred the cooked chicken and set aside.
Add the tomatillos, jalapenos, garlic, onion to a baking sheet and roast for 10-15 minutes at 400 degrees. Remove from oven when soft and blistered.
For the sauce, add the roasted tomatillos, jalapenos, garlic, onion, cilantro, chicken broth, and salt to a blender. Process until smooth.
Pour the sauce into a medium saucepan and simmer over medium heat for approximately 15-20 minutes until slightly thickened.
In the meantime, warm up each tortilla on a hot cast iron skillet for about 10 seconds on each side to make them pliable.
To assemble the enchiladas, dip each tortilla into the green chili sauce.
Fill each tortilla with the shredded chicken and a handful of cheese, roll them up, and place seam-side down in the baking dish.
Cover the enchiladas with the remaining green chili sauce and top with remaining cheese and chopped onions.
Bake for 15-20 minutes until the cheese is bubbly and lightly browned.
Garnish with fresh cilantro and serve hot.