The sauce. Toast the chiles until fragrant, about 30 sec on each side. Remove stems and seeds. Place in a bowl and cover with one cup of boiling water.
In a medium saucepan, heat the oil, add the chopped tomatoes, chopped onion, and garlic, and cook until soft. Put the ingredients in the blender container, and add the chiles and the soaking water. Blend until smooth. Return to the saucepan, add the oregano, cumin, and salt, and simmer until thick and fragrant. About 7-10 minutes.
The filling. Add the oil to a frying pan over medium-high heat, add the onion and garlic and sauté until fragrant. Add the mushrooms and let them cook for about 3 minutes; add the corn and zucchini. Season with salt. Let the vegetables cook.
The tortillas. Heat the tortillas over a lightly oiled hot pan or comal.
Assemble the casserole. Spoon about half a cup of guajillo sauce on the bottom of the baking dish, and place 8 tortillas to cover the bottom of the pan. Cover the tortillas with the vegetables, add 2 cups of sauce, cover the vegetables with tortillas, and spoon sauce ontop ot the tortillas, just enough sauce to cover it.
Finish with the shredded cheese and bake for 15-20 at a 375ºF oven. The cheese should be melted and the sauce bubbling.