Preheat the oven to 400 degrees Fahrenheit with racks in the middle and upper third of the oven. Line two large, rimmed baking sheets with parchment paper.

On one pan, combine the cauliflower and sweet potato. On the other pan, combine the bell peppers and onion. Drizzle half of the olive oil over one pan, and the other half over the other pan. Same with the cumin.

Sprinkle both pans lightly with salt and pepper, then toss each one until the vegetables are lightly coated in oil and spices. Arrange the vegetables in an even layer across each pan. Bake until the vegetables are tender and caramelized on the edges, about 30 to 35 minutes, tossing the veggies and swapping the pans halfway.

Reduce the oven heat to 350 degrees Fahrenheit and lightly grease a 9" square baker. Stir the cilantro into the salsa.

Spread ½ cup salsa evenly over the bottom of the baking pan. Add a single layer of halved tortilla pieces, arranging them to completely cover the salsa.

Top with ½ of the beans, ⅓ of the vegetables, ½ of the spinach, and ⅓ of the cheese.

Make a second layer of tortillas and press down on the mixture a little to make room for the next layers. Top with ½ of the remaining salsa, all of the remaining beans, ½ of the remaining vegetables, all of the remaining spinach, and ½ of the remaining cheese.

Make a third layer of tortillas and press down again to make more room. Top with the remaining salsa, vegetables, and cheese.

Cover the pan with parchment paper or aluminum foil. Bake for 20 minutes, then remove the parchment paper or aluminum foil and bake for 10 more minutes, or until heated through.

Let the casserole cool for 10 minutes before serving. Sprinkle the top lightly with additional chopped cilantro.