Slice portobello mushroom caps into pointer finger width slices. Heat oil in a pan over medium heat, then add sliced portobello mushrooms. Cook for 5 minutes, until softened.
Add the barbecue sauce to the mushrooms. Bring the sauce to a simmer, cooking for another 5-10 minutes. The mushrooms will absorb the BBQ flavor. Cook the mushrooms longer (up to 30 minutes) for a stronger BBQ infused flavor.
While the mushrooms are cooking, prepare the coleslaw. Add all coleslaw ingredients to a small bowl, except the coleslaw mix. Stir to combine. Drizzle the sauce over the coleslaw mix, tossing to combine.
Prepare corn tortillas by heating them in an unoiled pan over medium high heat. Flip after they are lightly browned and starting to puff (about one minute).
Divide the BBQ pulled portobellos and spicy mustard coleslaw evenly between the 8 corn tortillas. Top with additional pumpkin seeds and optional toppings, such as avocado, diced tomatoes and onions, and cilantro. Enjoy!