1In large skillet, melt butter over medium heat. Cook onion in butter about 15 minutes, stirring occasionally, until golden. Add mushrooms and taco seasoning mix. Cook 3 to 5 minutes longer, stirring occasionally, until onion and mushrooms are soft. Remove from heat; cover to keep warm.
2Wrap tortillas in microwavable paper towel. Microwave on High about 20 seconds or until warm. For each taco, stack 2 tortillas; spoon mushroom-onion mixture evenly over tortillas. Sprinkle with cilantro and cheese.