In a lightly oiled shallow 3 1/2- to 4-quart casserole, arrange 4 tortillas so they cover the bottom. Stack remaining tortillas and cut into 3/4-inch-wide strips; set aside. Mix tomatoes and juice with chili powder and cumin. Pour over whole tortillas. Top tomatoes with corn, then jalapeños, bell pepper, 1/2 cup onions, beans, and cilantro. Cover with 1/2 cup water.
Arrange tortilla strips evenly over filling, then sprinkle with cheese. Cover casserole with foil. Bake in a 400° oven until hot, about 20 minutes. Uncover casserole and bake until cheese is completely melted, about 5 more minutes. Sprinkle with remaining green onions just before serving.