How to Make It
Add the onion, tomatillos, Serrano chiles, garlic and cilantro to a blender carafe and process until smooth. Season with salt. Add the avocado and process until incorporated but still chunky.
In a medium skillet over moderate heat, coat the bottom of the pan with the olive oil. When warm but not sizzling, add the garlic and cook until fragrant and just beginning to brown. Add the mushrooms and cook until softened. Season with salt. Remove from heat and toss with fresh parsley. Fill each tortilla with the cooked mushrooms, top with guacamolada, and serve immediately.