"This taco filling with mushrooms, pieces of caramelized tomatillos, and a touch of chile de arbol tastes delicious even without the tortillas as a side dish. Stuff it into corn tortillas for a great vegan taco."
Set oven rack about 6 inches from the heat source and preheat the oven's broiler. Line a baking sheet with aluminum foil. Place tomatillos on top, cut-side up.
Broil in the preheated oven until they have a dark brown color, about 20 minutes. Remove from oven and allow to cool slightly.
Heat oil in a large skillet over medium heat while tomatillos cool. Add mushrooms and cook, stirring frequently, until softened, about 5 minutes.
Grate 2 or 3 roasted tomatillos and chiles into a bowl. Pour over mushrooms in the skillet. Add remaining tomatillos with the roasting juices from the baking sheet. Mix well and cook over low heat until mushrooms have softened, 5 to 8 minutes. Season with salt.
Warm corn tortillas in a skillet. Divide mushroom mixture amongst tortillas and top with chopped onion and cilantro.