Heat a grill to medium heat with a 12-inch cast-iron skillet (or grill pan) inside.

Arrange the peppers on a tray and the mushrooms and onion wedges on a separate rimmed plate. In a small bowl, whisk together the avocado oil, garlic, chili powder, cumin, salt, and several grinds of pepper. Drizzle 1 scant tablespoon of the mixture over the peppers and toss to coat. Drizzle the remaining marinade over the mushrooms and onions. Drizzle the mushrooms with a splash of balsamic vinegar and use your hands to coat the mushrooms on both sides.

Grill the peppers in the cast-iron pan for 8 to 10 minutes, tossing occasionally, until charred and soft. Be careful not to overfill the pan or the veggies won’t char on the edges - cook them in 2 batches, if necessary. Grill the mushrooms and onion wedges directly on the grill for about 4 minutes about per side. Remove everything from the grill, using a potholder to remove the cast-iron pan. Squeeze the juice of 1/2 lime over the peppers and season with salt, pepper, and more chili powder, to taste. Slice the mushrooms into strips and place on a serving dish. Transfer the onions and the peppers to a serving skillet or large platter.

Serve with the tortillas, guacamole, jalapeños, tomatoes, cilantro, salsa, and remaining lime wedges for serving.