Preheat the oven to 425 degrees F. Lightly oil a large shallow roasting pan or rimmed cookie sheet.
Place red bell peppers, cauliflower, sweet potato, onion, corn kernels and poblanos onto cookie sheet. Drizzle olive oil and sprinkle the cumin and minced garlic over top. Add a generous pinch or two of salt and black pepper, then use your hands to mix everything together. If you have a smaller pan, mix them in a bowl and then pour onto pan. After everything is coated well, spread the vegetables evenly in the pan. Roast for 30-40 minutes until vegetables are tender and begin to brown in spots. Stir or shake the pan every 10 minutes for even roasting. Remove pan from oven and reduce oven temperature to 350 degrees F.
Prepare an 8x8x2 or 9x9x2-inch square baking pan with nonstick spray. In a small bowl, stir the cilantro into the salsa. Spread 1/4 cup of salsa into the bottom of the baking pan. Add a layer of tortilla pieces, to completely cover the salsa. Top with 1/3 of the vegetables, a handful of spinach, and 1/3 of the cheese. Make a second layer of tortilla, salsa, vegetables, spinach, and cheese. Top with with a layer of tortillas, salsa, vegetables, and cheese. Cover with aluminum foil.
Bake for 20 minutes. Remove the foil and bake another 10 minutes, until cheese is melted and everything is heated through.
Let it sit for 5 minutes and cut into squares. Serve with sour cream and a sprinkle of sliced scallions.