AZTECA SAUCE: To make sauce, combine tomatoes and chilies in a blender and make a coarse puree. Heat the oil in a skillet over medium heat. Add onion and cook until lightly browned. Increase heat to medium-high, add garlic and cook 1 minute. Add tomato and chili mixture and cook, stirring frequently, until reduced and thickened, about 8 minutes. Add stock, reduce heat to low and simmer, uncovered, stirring occasionally, for 30 minutes. Stir in salt, remove from heat and stir in 1/4 cup of cilantro. Makes 4 cups of sauce.
CASSEROLE: Preheat oven to 350 degrees. Spread tortillas out on two baking sheets coated with vegetable oil spray. Lightly coat tops of tortillas with vegetable oil spray and place them in preheated oven for 10 minutes. Turn over and bake 3 minutes more. Remove from oven and set aside, leaving oven on.
Steam diced zucchini until just crisp-tender, about 2 minutes. Remove from steamer and set aside.
To assemble casserole, lightly coat a 7-by-11 inch baking dish with vegetable oil spray. Spread a thin layer of sauce evenly over the bottom.
Cover sauce with 2 toasted tortillas. Evenly spread the chopped spinich over the top of the tortillas. Top with one-fourth of the remaining sauce and one-fourth of the cheese.
Place 2 more tortillas over the top and press down. Spread the corn on top and cover with one-third of the remaining sauce and one-third of the remaining cheese. Top with 2 more tortillas, again pressing down. Top with zucchini, one-half the remaining sauce and one-half the remaining cheese. Top with the 2 remaining tortillas and press down. Spread remaining sauce evenly over the top and sprinkle on remaining cheese.
Cover tightly with foil and place in preheated oven for 25 minutes. Uncover and bake about 10 minutes more, or until lightly browned. To serve, sprinkle 1/4 cup of chopped cilantro over the top.