6 tablespoons olive oil
1 1/4 pounds white button or brown mushrooms, cleaned and chopped
12 cups shredded lacinato or flat-leaf kale leaves, center ribs removed (from about 1 to 2 large bunches)
1 tablespoon ground cumin
15 (6-inch) corn tortillas
2 cups salsa, plus extra for serving
Salt and pepper
Optional toppings: diced avocado, cilantro
Preheat the oven to 350°F. Coat a 9x13-inch baking dish with cooking spray.
Heat 4 tablespoons olive oil over medium heat in a large sauté pan. Add the mushrooms, season with 1/2 teaspoon of salt and a pinch of ground pepper, and stir to coat with the oil. Cook, stirring occasionally, until the mushrooms are soft, about 10 minutes.
Pour in the remaining 2 tablespoons of olive oil and stir in the kale. Season with cumin, 1/2 teaspoon of salt, and a pinch of ground pepper. Cook until the kale is wilted and soft, 3 to 5 minutes.
To assemble the casserole, arrange 5 of the tortillas in a single layer in the bottom of the baking dish. Spread 2/3 cup of the salsa over the tortillas, then top with half of the kale and mushroom mixture. Top with another layer of 5 tortillas, then spread with another 2/3 cup of the salsa, and top with the remaining kale and mushrooms. Top with the last 5 tortillas and spread the remaining salsa across the top of the tortillas.
Cover the baking dish with foil and bake for 20 minutes, until heated through. Remove from the oven. Let the casserole sit for a few minutes before serving with toppings, if using.