Heat oven to 325°F. Lightly grease 12x8-inch (2-quart) baking dish. Peel and shred into cold water enough potatoes to make 6 cups. Drain thoroughly; pat dry with paper towels.
In large bowl, combine potatoes, carrots, onion, half-and-half, salt and pepper; mix well. Spoon into greased baking dish.
Melt margarine in small saucepan over low heat. Stir in bread crumbs andparsley. Sprinkle over potato mixture.
Bake at 325°F. for 60 to 70 minutes or until vegetables are tender. Let stand 5 minutes before serving.