3/4 pound shredded mild melting cheese, such as Monterey Jack (about 3 cups)
15 to 16 ounces ricotta cheese
About 1/3 cup coarsely chopped fresh cilantro, for garnish
Heat the oil in a large skillet or enameled cast-iron casserole. Add the mushrooms and sauté over moderate heat until almost tender, about 3 minutes. Add the corn and sauté for 2 minutes. Transfer to a large plate to cool.
Add the spinach with the water that clings to the leaves to the skillet and cook over moderate heat, stirring constantly, until milted, 1 to 2 minutes. Transfer to a colander and press out the extra liquid, then add to the mushrooms and corn. Season with salt.
In a small skillet, heat the vegetable oil over moderate heat. Fry the tortillas one at a time, turning once, just until softened, a few seconds per side. Drain the tortillas in a single layer on a paper towel-lined baking sheet, blotting them dry with additional towels.
Preheat the oven to 350°. Lightly butter a 13-by-9-inch nonreactive baking dish. mix the Roasted Tomatillo-Chipotle Salsa with the cream in a bowl and spread 1 scant cup in the prepared dish. Arrange 8 tortilla halves in a single layer on the salsa. Top with half of the vegetables, 1 1/2 cups of the creamy tomatillo salsa, 1 cup of the cheese and 8 tortilla halves. Spread the ricotta on the tortillas and cover with 1 1/2 cups of the salsa. Top with 8 tortilla halves, the remaining vegetables, 1 1/2 cups of the salsa and 1 cup of the cheese. Finish with the remaining 8 tortilla haves, salsa and cheese.
Cover the dish loosely with foil and bake for 25 minutes. uncover andbake for about 25 minutes longer, or until bubbling and lightly browned. Let stand for at least 10 minutes. Sprinkle with the cilantro and serve.
The casserole can be refrigerated for up to 1 day. Rewarm in a 350° oven before serving.