Serve these as a vegetarian meal, or with meat-based tacos to mix things up.
Toss together onion and vinegar in a microwavable dish. Microwave on High for 1 minute; set aside.
Bring beer to a boil in a small saucepan over high; cook until reduced to ½ cup, about 5 minutes. Stir in honey.
Heat oil in a large skillet over high. Add cauliflower and mushrooms. Cook, stirring often, until caramelized and softened, 6 to 8 minutes. Add salt, cumin, and chile powder; cook, stirring often, until fragrant, about 1 minute. Add beer mixture and cilantro; cook, stirring occasionally, until vegetables are tender, about 2 minutes.
Divide vegetable mixture among tortillas. Gently stir onion mixture, and drain any liquid. Top tacos with pickled onion, and garnish with Cotija, pico de gallo, and avocado, if desired. Serve with lime wedges.