In a large saucepot, bring chopped sweet potatoes and 6 cups of water to a boil. Cook sweet potatoes until tender (soft enough for a fork to easily pierce). Strain off water. In a large skillet, bring 1 cup of vegetable oil to almost a smoking heat and add 3 cups of fresh sage to fry. Remove and drain on paper towel. Once dry, finely chop sage and set aside. In a large bowl, mash sweet potatoes with brown sugar and butter. Fold in chopped sage and season with salt and pepper to taste. Rub lamb chops with olive oil and salt and pepper, and grill to desired temperature and doneness. To plate: Lay three separate scoops of sweet potato mash on a long plate. Stand grilled lamb chops upright with bones facing all the same way. Take one winter sorrel leaf and garnish each mound of sweet potato. Serve with either a demiglace or brown gravy.