For the enchilada sauce: Heat a large cast-iron skillet over high heat. Remove the stems from the chiles, cut a slit up one side of each, and remove the seeds. Open up flat. One at a time, press the opened chiles flat onto the hot skillet until blistered and light brown. Turn and repeat, being careful not to burn.
Place the chiles in a small, deep bowl and add 1 cup of the hot water. Soak the chiles for 45 minutes, occasionally pressing them down into the water. Place the chiles and soaking liquid in a blender. Add the garlic and salt and run the blender on high speed for 2 to 3 minutes, until very smooth.
In a saucepan, heat the oil over medium heat. Add the onion and cook and stir until soft. Add the chiles. It will splatter, so have a lid or splatter screen ready. Fry the sauce for 7 minutes, stirring occasionally. Rinse the blender with the remaining 1/2 cup of hot water, add it to the salsa, and simmer for 10 minutes. If desired, strain through a food mill or coarse sieve. I like the salsa to be thick enough to cling, but you may thin with more water if desired.
For the enchiladas: Heat the oil in a large saute pan over medium-high heat. Add the mushrooms and onions and saute until softened and mushrooms are lightly golden. Add the black beans, cumin, coriander, and season to taste with salt. Saute for a few more minutes until beans are heated through to allow flavors to meld. Remove from the heat and cool completely. When cooled, mix in 3 ounces of the shredded cheese and set aside.
Preheat oven to 400 degrees F and lightly grease a large baking dish. Gently warm tortillas so they are soft and pliable (I like to put them on a plate, cover with a damp paper towel, and microwave for about 10 to 15 seconds). Fill each corn tortilla with about 1/3 cup filling, rolling the tortilla carefully but tightly to encase filling. Set seam side down in the prepared baking dish. Repeat with remaining tortillas and filling. Spoon sauce over the enchiladas and top with the remaining 4 ounces shredded cheese.
Bake 15 to 20 minutes, or until cheese is bubbly and edges are crusty. Serve immediately.