Bring a large pot of salted water to a boil over high heat. Once boiling, add potatoes. Cook for 15 minutes, until tender. Drain potatoes and add them back to the large pot. Mash until they reach desired consistency. Stir in half and half, butter, truffle oil, cheese, salt and pepper. Keep warm over low heat.

In a medium saucepan, melt butter for the sauce over medium-high heat. When melted, add shallots and saute for approximately 3 minutes. Stir in brandy, rosemary, and pepper. Cook down for approximately 2 minutes. Stir in port and bring to a simmer. Add beef stock and bring to a boil. Reduce to a simmer and cook for approximately 20 minutes. Strain and keep warm over low heat.

Heat a large cast iron skillet over medium-high heat. Season steaks with salt and pepper. Add butter to the pan and melt. Add steaks and sear on all sides, approximately 1 to 2 minutes per side. Cook to desired doneness, approximately 3 to 5 minutes per side.

Serve steaks over truffle mashed potatoes and top with port sauce. Garnish with parsley, if desired.