Season the spinach with salt/pepper and wilt in olive oil in a large skillet or pot over high heat. Cool and drain, squeeze excess liquid using a dish towel or cheesecloth. Lay the 6oz Filet Mignon on a flat surface and top each with spinach. Shred the potatoes, press out excess liquid and season with salt and pepper. Squeeze dry, then place in a bowl and toss with the horseradish sauce. Lay a 12-inch square piece of parchment paper on the work surface. Down the center of the parchment, place a thin layer of shredded potato. Lay a spinach-topped 6oz Filet Mignon on top of the potato and top with more shredded potato to cover. Pull up the parchment to completely envelope the tenderloin in the shredded potato. Repeat with the remaining 6oz Filet Mignon. Remove the parchment and immediately place the tenderloins in a hot skillet. Brown on all sides. Put the beef in a 400 degree Fahrenheit oven and cook to desired doneness. Remove from oven and reserve. Meanwhile for the sauce, combing the cabernet sauvignon, shallots, thyme, and peppercorns in a small pan over medium-high heat. Reduce mixture by 3/4. Add the stock and simmer over low heat. Strain. Just before serving, whisk in the butter.