Remove the stems and gills from the mushrooms: Cut the stems off completely off from the base of the portobella mushroom cap if they're still attached. Then, remove the dark, flaky gills by holding the mushroom in one hand and sticking the tip of a spoon into the gills with the other hand. Pry them up and away from the base as best you can. You should then be able to slide the side of the spoon all the way down and remove the gills in larger sections. (Do the best you can here. They don't have to be completely and cleanly removed.)
Quickly marinate mushrooms: Slice the mushrooms into long strips and transfer them to a large mixing bowl. Add the olive oil, lime juice, cilantro, garlic, chile powder, salt, cumin, and oregano. Toss to combine.
Cook the mushrooms: Heat a large nonstick skillet over medium-high heat. Add the marinated mushrooms and cook for 10 minutes, stirring occasionally. Transfer the mushrooms to a medium bowl or plate and set aside.
While the mushrooms are cooking, make the slaw: In a large mixing bowl, add the cabbage, mayonnaise and cilantro. Toss to combine. Refrigerate until ready to assemble tacos.
Cook the peppers: In the same skillet, add 1 tablespoon of olive oil and the bell peppers. Cook for 10 minutes, stirring occasionally, until peppers have softened.
Bring it together: Add the mushrooms back to the skillet, stir to combine with the peppers and cook them for 1 minute to warm everything back up.
Assemble the tacos: Fill warm corn or flour tortillas with a handful of slaw and the mushrooms and peppers. Serve immediately.
To grill the portabella mushrooms instead of sauteing them, remove the stem of the mushrooms but leave the gills on. Toss the mushroom caps together with the marinade and grill over high heat for 4-5 minutes per side, until beautiful grill marks appear and the mushrooms begin to caramelize. Remove from grill and slice into strips.
Don't have portobella mushrooms? Use cremini mushrooms (also called 'baby bella') instead. The average portobella mushroom is between 3 to 5 ounces, so I recommend getting around 18 to 30 ounces of cremini mushrooms.
To make this recipe vegan, use a vegan mayonnaise like Vegenaise.
To make this a little spicy, add in 1 to 2 teaspoons of crushed red pepper flakes to the marinade.