Preheat the oven to 425 degrees F. Remove most the outer fat from the lamb; cut into 2, 4-rib racks. In a very hot, ovenproof, non-stick skillet, sear the lamb for 1 minute on each side, starting with the meat-side. Remove from the heat and drain any excess fat that has melted from the lamb. Brush the meat side of the lamb with the mint sauce; season with salt and pepper and top with walnuts. Roast 22-25 minutes for rare, 25-28 for medium rare and 30 for medium. Set the lamb on a plate, tent with foil and rest 10 minutes. Place the skillet on the stovetop over medium-high. Add the stock/flour mixture, tomato sauce and paprika; gentle simmer 5 minutes. Whisk in the butter; season with salt and pepper. Serve the sauce alongside the lamb for drizzling on top.