Cut poblanos and bell pepper in half lengthwise; discard seeds and membranes. Place peppers, skin side 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 peppers aside separately.
Place flour, ½ teaspoon salt, and black pepper in a large saucepan. Gradually add milk and broth, stirring with a whisk. Cook over medium heat until slightly thick (about 12 minutes), stirring constantly. Remove from heat. Combine milk mixture and poblanos in a blender; process until smooth. Set aside.
Preheat oven to 350° F.
In a medium bowl, combine chopped roasted bell pepper, corn, remaining ½ teaspoon salt, 1 cup cheese, red onion, cilantro, eggs, and ricotta.
Coat bottom of a 13 x 9-inch baking dish with oil or cooking spray. Spread ½ cup sauce in bottom of dish. Arrange 6 tortillas over sauce, overlapping slightly. Spread half of ricotta mixture over tortillas; top with half of white beans. Sprinkle with 1/3 cup green onions and 2/3 cup cheese. Pour about 1 cup sauce over cheese. Repeat layers with 6 tortillas, remaining ricotta mixture, remaining beans, 1/3 cup green onions, 1 cup sauce, and 6 tortillas. Pour remaining sauce over tortillas. Coat 1 side of foil with oil or cooking spray. Place foil, coated side down, over casserole. Bake at 350° F for 30 minutes or until bubbly.
Uncover; sprinkle with remaining cheese. Bake, uncovered, 15 minutes or until cheese melts. Let stand 15 minutes. Garnish with remaining green onions, sliced avocado, and hot sauce as desired. Enjoy!