First, make the cilantro chimichurri sauce. Add the cilantro, parsley, olive oil, garlic cloves, red wine vinegar, lime juice, salt, and optional chili flakes to a food processor or blend. Pulse a few times and pulse to blend together. Then, pour into a bowl and set aside.
Next, slice the portobello mushrooms into ½-1" thick slices and place into a large bowl. If you haven't done so already, now would be a good time to slice any other vegetables like the shredded cabbage and set aside for later. In a separate small bowl, whisk together the seasoning ingredients including the chili powder, garlic powder, salt, white pepper, cumin, avocado oil, and lime juice. Then, pour the seasoning mixture into the bowl of sliced mushrooms and use your hands to gently coat the mushrooms with the seasoning.
Lightly coat a pan with oil and turn on the heat. Add the mushrooms to the pan in a single layer and cook for 3-5 minutes on each side until the mushrooms have browned. It's important to do this in a single layer, so it will likely take two rounds of cooking. Alternatively, preheat the oven to 400F and add the mushroom slices to a parchment lined baking sheet in a single layer. Then, cook them for 10-12 minutes until browned. Remove the mushrooms from the heat and set aside.
In a small bowl mash together the avocado, lime, salt & pepper to make an easy avocado smash. Add 1 tbsp of avocado smash to each taco, top with 2-3 portobello mushrooms, shredded cabbage, and about 1-2 tsp of cilantro chimichurri. Serve warm and enjoy!