Preheat a grill or grill pan over medium-high heat until hot.
Clean the portobello mushrooms and grill them until tender and slightly charred, about 4–6 minutes per side depending on thickness.
Remove the mushrooms from the grill and slice them into thin strips.
If using, brush the sliced mushrooms lightly with adobo sauce to taste for a spicy kick.
Warm the tortillas on the grill or in a dry skillet until pliable, about 30–60 seconds per side.
Assemble each taco by layering grilled mushroom strips, avocado salsa, and pickled red onions on a warm tortilla, then sprinkle with chopped fresh cilantro.
Serve the tacos with lime wedges for squeezing over the top.