Preheat broiler. Lightly coat a 2 quart square baking dish with cooking spray; set aside. Line a 15x10x1 inch baking pan with foil. Place tomatillos in baking pan. Broil 4 to 5 inches from heat for 6 to 8 minutes or until softened and charred, turning occasionally. Set aside to cool slightly.
In a large skillet heat oil over medium heat. Add onion and chile pepper; cook and stir for 4 to 5 minutes or until tender and onion starts to brown.
In a blender or food processor combine tomatillos, onion mixture, the ¼ cup cilantro, the sugar, cumin, and salt. Cover and blend until smooth, stopping and scraping sides as necessary.
Spread ¾ cup of the tomatillo mixture in the prepared baking dish. Arrange 6 tortilla halves over the sauce, overlapping slightly. Top with 1 cup of the chicken, ½ cup of the cheese, and half of the remaining tomatillo mixture. Add six more tortilla halves, the remaining 1 cup chicken, ½ cup cheese, and half of the remaining tomatillo mixture. Top with the remaining six tortilla halves and the remaining salsa, spreading to cover.
Cover the dish with plastic wrap and chill for at least 8 hours (up to 24).