Preheat the oven to 400 degrees, and spray a 9 x 13-inch baking dish with nonstick cooking spray.
Heat a large heavy skillet over medium to medium-high heat, and when it is hot, add the oil and swirl it around. Add the zucchini, onions, and peppers and sauté them, stirring occasionally, until they are tender and starting to brown in spots, about 10 minutes.
Meanwhile, combine the salsa and pumpkin in a blender and puree them. If it is too thick to pour out of the blender, blend in 1/2 cup water or broth to thin it.
Pour 1 cup of the sauce in the bottom of the baking dish and spread it around with a spatula. Top it with 6 tortillas, letting them overlap where necessary. Top the tortillas with 1/2 the sautéed vegetables, ½ the beans, 1/2 the remaining sauce, and ½ the cheese. Repeat the layers with the remaining tortillas, vegetables, beans, and sauce, ending with a layer of cheese. (At this point you can cover and refrigerate the casserole for up to 24 hours, if desired.)
Spray the bottom of a piece of foil, cover the dish with the sprayed side down, and bake it for 25 minutes. Remove the foil and bake it for 5 more minutes. Let the casserole cool for 10 minutes, if possible, cut it into squares to serve it, topped with sour cream, if desired, or refrigerate it for up to 3 days, or freeze it for up to 3 months.