Put the potatoes in a large pot and cover them with water. Add 1 teaspoon salt for each quart of water. Heat to boiling and cook for 20 to 25 minutes, until the potatoes are tender. Drain, peel, and mash the cooked potatoes.
Meanwhile, pare the rutabaga and cut it into 1-inch pieces. Place in a saucepan, cover with water, and bring to a boil. Simmer until tender, 25 to 30 minutes. Drain, mash, and add the cooked rutabaga to the potatoes. Beat with an electric mixer until the potatoes and rutabagas are smooth and fluffy. Beat in the flour, eggs, salt, and milk or cream.
Preheat oven to 350°F Butter a 3-quart shallow baking dish. Turn the mixture into the dish. Using a spoon, spread out the mixture, making indentations in the top of the casserole. Dot with the butter and sprinkle with the bread crumbs. Bake uncovered for 1 hour, or until lightly browned. 12 servings.