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.