Peel and cut potatoes into ¼-inch thick sticks. Soak in water for a minimum of 2 hours. Remove the frites from the water and dry very well.
Heat the oil in a deep pot until it reaches 330 degrees F. Fry potatoes in small batches in oil for about 3 minutes or until just cooked through but not browned. Drain on paper towels and cool to room temperature. Heat the oil to 375 degrees F. Fry potatoes again about 1 to 2 minutes or until brown and crispy. Drain on paper towels. Season the frites immediately with salt and pepper.
Season steak with black pepper and salt on both sides. Heat vegetable oil in a cast iron skillet over medium high heat until very hot. Sear steaks 5 minutes per side until well browned but medium rare. Remove steak from skillet. Set aside and keep warm. Reduce heat to medium and add butter to the skillet. Drain green peppercorns from the brining liquid. Lightly crush green and pink peppercorns with a mortar and pestle and add to skillet. Stir in mustard, cream and brandy. Pour sauce over steaks and serve with frites.