1. In a large bowl, whisk together olive oil, lime juice, garlic, jalapeno, cilantro, salt and pepper.
2. Wipe the portobellos with a damp towel. Pull out the stem and use a butter knife or spoon to scrape out the gills. Slice into ½-inch thick slices and add to the marinade bowl.
3. Add the sliced bell peppers and onions to the bowl and toss gently with your hands to combine.
4. Heat a large pot over medium-high heat. Pour in the vegetables and marinade. Cook, stirring occasionally, until vegetables are tender, about 8 minutes.
5. Meanwhile, pit and peel the avocado. Smash it in a bowl and mix in a little lime juice and a pinch of salt, if desired.
6. Serve vegetables in warmed tortillas, with smashed avocado and black beans.