Preheat oven to 350 degrees F.
Melt butter in a large skillet and add onion and cook for 1 minute.
Add mushrooms, and cook until mushrooms begin to soften for about 5 minutes.
Place the cheese and chile over the cooked mushrooms.
Place skillet in the oven and bake uncovered for about 10 to 12 minutes or until cheese is completely melted and slightly golden brown on top.
Sprinkle with fresh cilantro.
Serve with corn or flour tortillas.
