For an impressive weeknight dinner ready in no time, put rotisserie chicken to work as a hearty enchilada filling. Essential to Mexican cooking, salsa verde imparts a citrusy heat that deepens the flavors of this classic dish.
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 and 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, queso fresco and any remaining cilantro.