Preheat broiler. Place chiles on a foil-lined baking sheet; broil 10 minutes or until blackened, turning occasionally. Place in a zip-top plastic bag; seal. Let stand 15 minutes. Peel chiles; cut in half lengthwise. Discard seeds and membranes; slice into strips to measure 2/3 cup.
Place a large skillet coated with cooking spray over medium heat until hot. Add 1 cup sliced onion; cook 10 minutes or until golden brown, stirring frequently. Stir in chicken and 1/3 cup chiles; spoon chicken mixture into a bowl.
Steam spinach leaves, covered, 5 minutes or until wilted. Place spinach in a colander, pressing with the back of a spoon until barely moist.
Preheat oven to 350°.
Wipe skillet with paper towels, and recoat with cooking spray. Place over medium-high heat. Add 1/2 cup sliced onion and chopped green onions, and saute 3 minutes. Stir in ground cumin and minced garlic, and saute 30 seconds. Combine masa harina and broth. Add masa mixture, chopped cilantro, cream cheese, and salt to pan, and bring to a boil. Reduce heat, and simmer 5 minutes or until cheese melts, stirring constantly. Place the cheese mixture, 1/3 cup chiles, and spinach ina food processor, and process until smooth. Strain spinach mixture through a colander into a bowl, and discard solids.
Divide the chicken mixture evenly among tortillas, and roll up. Pour 1/2 cup spinach sauce in bottom of an 8-inch square baking dish coated with cooking spray. Arrange the filled tortillas on top of spinach sauce. Pour the remaining spinach sauce over tortillas. Cover and bake at 350° for 10 minutes or until enchiladas are thoroughly heated.