1 1/2 pounds tomatillos, husked and rinsed
1 to 2 serrano chile peppers**, stemmed and seeded
1/2 cup low-sodium chicken broth
Pinch of sugar
3 cups shredded rotisserie chicken
2 1/2 cups shredded mozzarella and/or Monterey Jack cheese
1/3 cup fresh cilantro
2 tablespoons olive oil, plus more for greasing
3/4 cup crumbled feta cheese
*Note: Depending on the amount of heat you want, you can modify the pepper you use
Preheat the broiler. Slice half of the onion into thin rings and set aside.
Place the other onion half, tomatillos and serranos on a foil-lined baking sheet and broil until the vegetables are soft and slightly brown, 7 to 10 minutes, turning as needed. Transfer the vegetables and any liquid to a blender, add the broth and puree. Season with 1/2 teaspoon salt and the sugar.
Meanwhile, stack the tortillas, wrap in a damp paper towel and microwave just until warm and soft, 1 minute; keep covered. Toss the chicken with 2 cups shredded cheese in a bowl. Spoon a portion of the chicken mixture down the middle of each tortilla. Add a few cilantro leaves and roll up. Place the enchiladas side by side in a lightly oiled 9-by-13-inch baking dish brush with the 2 tablespoons olive oil. Broil until crisp and golden, 3 minutes.
Pour the tomatillo sauce over the enchiladas and top with the remaining 1/2 cup shredded cheese. Return to the oven and broil until the cheese is bubbly and golden brown, 3 to 5 minutes. Garnish with the onion rings and any remaining cilantro.