Nonstick cooking spray

1 pound tomatillos, husks removed and rinsed

1 teaspoon vegetable oil

½ cup onion, chopped

1 fresh poblano chile pepper, seeded and chopped

¼ cup fresh cilantro, snipped

1 teaspoon sugar

½ teaspoon ground cumin

¼ teaspoon salt

12 (6-inch) corn tortillas, halved

3 cups shredded cooked chicken breast

1¾ cups shredded reduced-fat Mexican-style four-cheese blend (7 ounces)

1 (16-ounce) jar salsa

Chopped tomato, onion, jalapeños, green onions, or cilantro (optional garnish)

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Preheat broiler and line a baking sheet with foil. Roast tomatillos 4–5 inches from heat until softened and charred, 6–8 minutes, turning occasionally. Cool slightly.
In a skillet, heat oil over medium heat. Add onion and poblano; cook 4–5 minutes until tender and browned.
Blend tomatillos, onion-poblano mixture, cilantro, sugar, cumin, and salt until smooth to make the sauce.
Grease a 2-quart baking dish. Spread ¾ cup sauce on the bottom.
Layer with 6 tortilla halves, 1 cup chicken, ½ cup cheese, and half the salsa. Repeat with 6 tortillas, 1 cup chicken, ½ cup cheese, and half the sauce. Add another 6 tortillas, 1 cup chicken, and remaining sauce. Finish with tortillas and remaining salsa.
Cover and refrigerate at least 8 hours or up to 24 hours. Chill remaining cheese separately.
Preheat oven to 375°F. Cover casserole with foil and bake 40 minutes. Remove foil, sprinkle with reserved cheese, and bake 20 minutes more until bubbly and hot. Let stand 10 minutes.
Garnish with tomato, onion, jalapeños, green onions, or cilantro, if desired.