Sprinkle veal shanks evenly with 1/4 teaspoon salt and pepper; dredge in flour, shaking off excess.
Heat 1 tablespoon oil in a Dutch oven over medium-high heat. Add veal shanks, and cook 5 minutes on each side or until browned. Remove veal from pan. Heat remaining 1 tablespoon oil in pan over medium-high heat. Add carrots, onion, celery, and 3 garlic cloves; sauté 6 minutes. Add remaining 1/4 teaspoon salt, Marsala, and broth; bring to a boil. Return veal to pan. Reduce heat, cover, and simmer 1 hour. Uncover and simmer 30 minutes.
While veal is simmering, combine parsley, lemon rind, and 2 garlic cloves. Place 1 veal shank in each of 4 shallow bowls; spoon about 3/4 cup sauce over each serving, and sprinkle each with 1 tablespoon gremolata.
Slow-Cooker Method: Place browned shanks in a 5-quart electric slow cooker; top with sautéed vegetables. Add remaining 1/4 teaspoon salt, Marsala, and broth. Cover with lid; cook on HIGH 1 hour. Reduce heat to LOW; cook 5 hours. Serve with gremolata.
Osso Buco (AW-soh BOO-koh), or braised veal shanks, is the ultimate in rustic Italian cooking.