Step 1

Heat a large cast iron skillet over medium high heat.  Add butter and onions. Cook until slightly softened, stirring frequently for about 10 minutes.

Step 2

Season with salt and pepper.  Add mushrooms, stirring for an additional 10 minutes.  Mushrooms will be very soft and starting to turn golden.  Remove from the pan and set aside, keep warm.

Step 3

Wipe pan clean and return to the heat.  Drizzle olive oil in the pan.

Step 4

Season both sides of the steak well with salt and pepper.  Place in the pan and cook for 8-10 minutes per side.

Step 5

Top the steak with slices of mozzarella cheese, and cover the pan and wait for the cheese to melt.

Step 6

Remove steaks from the pan and allow to rest for 5 minutes.

Step 7

Serve with mashed potatoes, top steak with reserved mushrooms and onions.