In a blender, combine the cilantro leaves, scallions, garlic, 2 tablespoons oil, lime juice, water, and cumin and blend well.
Transfer the marinade to a medium bowl and add the mushrooms, rubbing the marinade into the mushrooms. Set aside for 10 minutes.
Heat 2 tablespoons oil in a large skillet over medium heat. Add the peppers and onion. Cook, stirring, until crisp-tender, 5 to 8 minutes. Season with salt and black pepper.
Heat a grill pan, electric grill, or another skillet over medium-high heat. Grill the mushroom slices in batches until cooked through and grill marks appear, about 3 minutes per side. Season to taste with salt.
Heat the tortillas between 2 damp paper towels in the microwave for 1 minute or heat each tortilla in a large skillet over medium heat. Keep warm.
To assemble, fill the warm tortillas with mushrooms, peppers, onions, poblano sauce, and avocado slices.
Roasted Poblano Cream Sauce
Roast the poblano pepper in a dry skillet, turning every few minutes. When well blistered and mostly charred, transfer the poblano to a bowl and cover the bowl with plastic wrap to steam for ten minutes.
Peel the chile by rubbing the skin off. Remove the stem and seeds. Coarsely chop the pepper meat and set aside.
Grind the cashews in a blender until finely ground. Add the almond milk, lime juice, cilantro, oil, cumin, onion powder, garlic powder, and chopped poblano.
Blend the sauce as smoothly as possible, in intervals, if necessary, to avoid overheating the blender. Season with salt and black pepper to taste.