Preheat oven to 400°F; lightly coat a 9x13-inch backing dish with cooking spray.
Heat oil in a large nonstick skillet over medium-high heat. Add onion and cook. Stirring often, until starting to turn brown (about 5 min).
Stir in zucchini, beans, tomatoes, corn, cumin and salt. Cook stirring occasionally. (until veggies are heated through - about 3 minutes).
Scatter half of the tortilla pieces in the baking dish. Top with with about half of the vegetable mixture, enchilada sauce and half the cheese. Repeat with one more layer of tortillas, veggies, sauce, and cheese. Cover with foil.
Bake casserole 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).