Preheat oven to 400 F. Line large 18-by-13-inch baking sheet with parchment paper. Set aside.
In large bowl, toss mushrooms, onions and peppers with oil and taco seasoning. Distribute on baking sheet. Roast 25 minutes, tossing halfway through, until veggies are fork tender and edges are slightly browned.
Serve with warmed tortillas and garnish with cilantro. Top with avocado, salsa, hot sauce or lime, if desired.