Preheat oven to 400°F. Lightly coat a 9-by-13-inch baking dish with cooking spray. Meanwhile, make the sauce: combine the tomato sauce, vegetable broth, chili powders (all 3 of them), garlic powder, onion powder, and dried oregano in a medium sauce pan over medium heat. Whisk to combine, then turn the heat down to low and let simmer.
Heat olive oil in a large nonstick skillet over medium-high heat. Add onion and bell pepper and cook, stirring often, until starting to brown, about 5 minutes.
Stir in beans, tomatoes, corn, cumin and salt and cook, stirring occasionally, until the vegetables are heated through, about 3 minutes.
Scatter half the tortilla pieces in the casserole dish.
Top with half the vegetable mixture, then half the sauce, and then half the cheese.
Repeat with one more layer of tortillas, vegetables, sauce and cheese.
Cover the casserole with foil and bake for 15 minutes. Remove the foil and continue baking until the casserole is bubbling around the edges and the cheese is melted, about 10 minutes more.
Top with diced tomatoes and green onion.