Preheat oven to 350°F.
Prepare the Mushrooms Heat oil in a medium skillet over medium. Add onion and jalapeño; cook, stirring often, until translucent, about 2 minutes. Add garlic; cook, stirring often, until fragrant, about 30 seconds. Increase heat to high, and add mushrooms. Cook, stirring constantly, until lightly browned and the liquid from the mushrooms evaporates, about 4 minutes. Immediately transfer mushrooms to a bowl, and season with salt. Set aside. (Do not overcook the mushrooms.)
Prepare the Queso Toss cheeses together in a small cast-iron skillet or ovenproof dish; bake in preheated oven until cheese is completely melted, about 10 minutes. Remove from oven, and top with Mushrooms. Serve immediately with tortillas and salsa of your choice.
