Make the marinade. In a large bowl add the olive oil, lime juice (from 1 and 1/2 limes), garlic, jalapeño, cilantro, salt and pepper. Whisk together.
Wipe mushroom with a damp towel, to clean. Pull out the stem and use a spoon to scrap out the gills. Slice into 1/2-inch thick slices and add to the marinade bowl.
Add the sliced bell peppers and onions to the bowl and toss gently with your hands to combine.
Heat a small amount of olive oil or avocado oil in a large pot or cast-iron skillet over medium-high heat. Pour in the vegetables. Cook, stirring occasionally, until vegetables are tender, about 8 minutes.
Meanwhile, pit and peel the avocado. Smash it in a bowl and add in a little lime juice and a pinch of salt, if desired.
Serve vegetables in warmed tortillas, with smashed avocado (with remaining half lime juice) and black beans on the side.