Preheat the oven to 200 degrees F and line a baking sheet with paper towels.
Put the masa harina in a large bowl, pour in the water and get in there with your hands, kneading until you have a uniform dough. It should take only a minute or two. You want a nice, moist dough, so work in a little more warm water if it's dry. If it's sticky, work in a little more masa harina.
Break the dough into four equal pieces and roll each one into a ball. Put the balls on a plate or cutting board and cover them with plastic wrap to keep the dough from drying out. One at a time, roll them out with a rolling pin or clean wine bottle until they're a thin circle about 1/4-inch thick. Brush a little of the vegetable oil on a skillet or comal and heat it over medium-high heat until it's good and hot. Carefully add the first sope and cook it until little brown blisters appear on the bottom, about 45 seconds. Flip it over with a spatula and cook for another 30 to 45 seconds. Flip it over again, count to 10 and transfer to the towel-lined baking sheet. Keep it warm in the oven. Repeat with the remaining 3 sopes (adding a little more oil if you need to), and keep them in the oven until you're ready to serve them.
Pour a couple of tablespoons of the oil into a skillet and add the chile, garlic and onions. Set the skillet over medium heat and cook, stirring occasionally, until the onions are soft, about 5 minutes. Add the huitlacoche (or mushrooms) along with a good pinch of salt and cook, stirring occasionally, until the huitlacoche is heated through, about 5 minutes (or until the mushrooms are a little browned and cooked through, about 10 minutes). Take the mixture off the heat and stir in the cilantro.
Arrange your sopes on a platter. Cover with a thin layer of Roasted Tomato-Chile de Arbol Salsa, the huitlacoche or mushrooms, a drizzle of the crema fresca and a sprinkle of queso fresco. Serve the remaining salsa on the side.
Roasted Tomato-Chile de Arbol Salsa:
Preheat the boiler.
Put the tomatoes on a baking sheet and broil until the tomatoes are nice and charred, 10 to 12 minutes. Take the tomatoes out, let them cool just until you can handle them, slip off the skins and cut out the tough cores. Transfer the tomatoes to a big bowl (don't you dare forget the tomato juice that has leaked out and reduced to awesomeness on the baking sheet), and then roughly chop them.
While the tomatoes are broiling, heat a dry skillet over medium heat and toast the chiles (in batches, if necessary), flipping them over occasionally, until they just begin to smoke, about 5 minutes. Set them aside in a bowl.
Put the olive oil, garlic and onions in a saucepan, set it over medium heat and cook, stirring occasionally, until the onions are soft, about 7 minutes. Add the toasted chiles, tomatoes and 2 cups water, bring to a simmer and cook for another 12 minutes, so the flavors come together. Let it cool a bit.
Carefully transfer the mixture to a blender. Add the cilantro, salt and pepper and puree until the mixture is very smooth. Pour the mixture through a medium-mesh sieve into a bowl. Serve at room temperature or slightly chilled. Store the salsa in the refrigerator, tightly covered, for up to a week, or in the freezer for a month.