Preheat oven to 450° F. Sprinkle chops with salt, pepper and about half the rosemary. Heat oil in large cast-iron skillet over medium heat. Working in batches, sear 2 minutes each side; remove and set aside. Add mushrooms to skillet with more oil if needed. Sautée until soft, about 3 minutes. Sprinkle with remaining rosemary. Place chops on top, and place in oven and roast for 45 minutes, turning once or until chops are cooked and slightly pink in the centre. Remove chops and mushrooms from skillet. Place skillet on medium-high heat; add wine and vinegar. Bring to a boil, scraping up browned bits from the bottom of the skillet. Add tomato paste and stock and boil until mixture becomes slightly syrupy, about 5 minutes. Whirl in the Garlic Horseradish until blended. Remove from heat and beat in butter. Stir in mushrooms and chervil. Place pork chops on plate stacked with garlic mashed potatoes. Top with mushrooms and drizzle with sauce.