Cut poblanos and bell pepper in half lengthwise; discard seeds and membranes. Place poblanos and bell pepper, skin sides up, on a foil-lined baking sheet; flatten with hand. Place corn on baking sheet. Broil 10 minutes or until poblanos and bell pepper are blackened and corn is lightly browned. Place poblanos and bell pepper in a zip-top plastic bag; seal. Let stand 10 minutes. Peel and coarsely chop; set poblanos and bell pepper aside separately. Remove corn kernels from cobs.
Lightly spoon flour into a dry measuring cup; level with a knife. Place flour, 1/2 teaspoon salt, and black pepper in a large saucepan. Gradually add milk, stirring with a whisk. Cook over medium heat until slightly thick (about 12 minutes), stirring constantly. Remove from heat. Combine 1 cup milk mixture and poblanos in a blender; process until smooth. Stir pureed poblano mixture into remaining milk mixture.
Preheat oven to 350°.
Combine bell pepper, corn, remaining 1/2 teaspoon salt, 1 cup Mexican cheese, red onion, cilantro, eggs, and ricotta.
Coat bottom of a 13 x 9-inch baking dish with cooking spray. Spread 1/2 cup sauce in bottom of dish. Arrange 6 tortillas over sauce, overlapping slightly. Spread half of ricotta mixture over tortillas; top with half of chicken. Sprinkle with 1/3 cup green onions and 2/3 cup Mexican cheese. Pour about 1 cup sauce over cheese. Repeat layers with 6 tortillas, remaining ricotta mixture, remaining chicken, 1/3 cup green onions, 2/3 cup Mexican cheese, 1 cup sauce, and 6 tortillas. Pour remaining sauce over tortillas. Coat 1 side of foil with cooking spray. Place foil, coated side down, over casserole. Bake at 350° for 30 minutes or until bubbly.
Uncover; sprinkle with remaining 2/3 cup Mexican cheese and 1/3 cup green onions. Bake, uncovered, 15 minutes or until cheese melts. Let stand 15 minutes.
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