Melt butter in a saucepan over medium heat. Add mushrooms and cook until soft, 5 to 7 minutes. Drain excess butter and return to heat. Add taco seasoning and stir to combine, about 2 minutes more.
Serve each tortilla topped with a few tablespoons of mushroom mixture, shredded lettuce, tomato, onion, guacamole, sour cream, and taco sauce.
