Scoop the gills out of, and slice, your portobello mushrooms.
Place oil in a large frying pan on medium-high heat, add the peppers and onions, and allow those to cook for a few minutes. Then, add the garlic. Cook for a total of 5 minutes, stirring frequently.
Add the mushrooms, cumin, garlic powder, salt, and smoked paprika. Cook for 10 more minutes, stirring frequently.
Layer the mushroom mixture on top of the tortillas.
Top with sliced avocado and any of the optional toppings, and serve.