How to Make It
Preheat the oven to 400°. Heat a grill pan or a large cast-iron skillet. Grill the tomatillos and unpeeled garlic clove over moderately high heat, turning often, until the tomatillos and garlic are soft and blackened in spots, 8 to 10 minutes. Add the chiles to the grill and toast just until lightly charred, about 3 minutes. Transfer to a plate and let cool slightly. Coarsely chop the chiles.
Remove the papery skin from the garlic clove. Transfer the garlic to a blender, add the grilled tomatillos and chiles, the water and 1/2 teaspoon of the salt and blend to a coarse sauce.
In a large skillet, heat 1 tablespoon of the oil. Add the onion, minced garlic, oregano and the remaining 1/2 teaspoon salt and cook over moderately high heat until the onion has softened, about 5 minutes. Add the zucchini and cook, stirring, until softened, about 5 minutes longer. Stir in the corn and cook just until heated through, about 1 minute.
Pour half of the tomatillo sauce into a 13-by-9-inch baking dish. In a medium skillet, heat the remaining 3 tablespoons of oil over moderately high heat. Using tongs and working with 1 tortilla at a time, dip both sides of each one briefly in the hot oil just until the tortilla softens; let any excess oil drip back into the pan. Lay the tortillas on a work surface and spoon about 21/2 tablespoons of the zucchini filling across the center of each; sprinkle each one with 1 tablespoon of the cheese. Roll up the tortillas and transfer them to the baking dish.
Pour the remaining tomatillo sauce over the enchiladas and sprinkle with the remaining cheese. Cover the dish with foil and bake for about 10 minutes, or until heated through. Serve immediately.