1Set a large, heavy skillet over medium-high heat. When it’s hot, add 1 tablespoon of the oil, then the onion and peppers. Season generously with salt and add the cumin and garlic powder. Cook, stirring frequently, until the vegetables have softened and lightly caramelized, about 10 minutes. Transfer to a warmed platter, loosely covered to stay warm. Set aside.
2Wipe out the skillet and return it to medium-high heat. Add the remaining 2 tablespoons of oil, swirl to coat, and add the mushrooms. Season with salt and cook, stirring frequently, until the mushroom slices have softened and are just beginning to sear, about 10 minutes. Transfer to the platter with the onion and peppers, cover loosely, and set aside.
3To make the guacamole, place the avocado in a small bowl and, with the back of a fork, mash it until it’s almost smooth. Add the lime juice. Season with salt and stir to combine.
4Serve right away with warm tortillas, guacamole, salsa, red onion, hot sauce, and cilantro.