Place the yams and ginger in a pot, cover with cold water and boil until the yams are tender. Meanwhile, heat the oil in large skillet set over medium-high heat. Season the lamb with salt and pepper. Sear 4 minutes per side, or until cooked medium rare to medium. Remove the lamb from the skillet and set aside. Add the port to the skillet, bring to a simmer, and reduce by half.
Add the stock/flour mixture and bring to a simmer. Return the lamb chops to the skillet and heat through a few minutes. Meanwhile, when tender, drain the yams and thoroughly mash. Mix in the butter, mint, salt and pepper. Divide and mound the mashed yams on 4 plates. Top with the lamb and sauce and serve, garnished with mint sprigs.