Sauté filling: Cook mushrooms in a dry skillet over medium heat until they release their liquid and it evaporates, about 5 min. Toss with taco seasoning and green chiles.
Build: Heat half the butter in skillet over medium. Place 1 tortilla, sprinkle half the cheese, mushroom mixture, remaining cheese, top with second tortilla.
Crisp: Cook 2-3 min until underside browns. Add remaining butter, flip, cook 2 min more until cheese melts.
Serve: Rest 1 min; cut into wedges. Add salsa & sour cream if desired.