1To make the sauce, in a large saucepan, add the tomatillos and garlic cloves and add enough water to cover. Bring to a boil over high heat, then reduce the heat to medium and simmer until the tomatillos soften and their color changes to pale green, about 10 minutes. Transfer the tomatillos and garlic to a blender with a slotted spoon. Reserve 1 cup of the cooking liquid. Add the serranos, cilantro, onion, and 1 tsp salt, and puree until smooth. Add some of the cooking liquid to thin the sauce if needed. Warm a large, deep frying pan over medium-high heat. Once it is hot, add the sauce. Simmer, stirring, until it thickens and deepens in color, 5 to 10 minutes. Season with salt and set aside.
2In a frying pan, warm the 1 tablespoon of olive oil. Add the 1/2 yellow onion and sauté until browned and tender. Add the mushrooms and cook, stirring, until just tender. Add the spinach and cook until wilted. Drain off any liquid. Add 1/4 cup of the tomatillo sauce and continue to simmer for 5 minutes for the flavors to meld. Remove from the heat and set aside.
3Preheat the oven to 375°F. Have ready a 9×13 dish. Spread 1 cup of the tomatillo sauce into the bottom of the dish.
4In the cast iron pan, warm the canola oil over medium-high heat. Fry the tortillas quickly and set aside to drain on paper towels. Blot dry.
5For each enchilada, add some of the spinach-mushroom mixture and a sprinkle of cheese, and roll up tightly. Place in the baking dish and repeat. (These can be prepped up to 1–2 hours ahead.) You should easily fit 12.
6Warm the remaining sauce. Pour the sauce over the enchiladas to generously coat them, then top with the remaining cheese. Bake, uncovered, until bubbly and heated through and the cheese is golden brown, 20 to 30 minutes. Let sit for 5 minutes before garnishing and serving.