Preheat oven to 375 degrees F (190 degrees C).
Place turnip and potatoes in a large pot with enough water to cover, and bring to a boil. Cook 25 to 30 minutes, until tender. Remove from heat, and drain.
Mix milk, 2 tablespoons butter, and sugar with the turnip and potatoes. Season with salt and pepper. Mash until slightly lumpy.
Transfer turnip mixture to a small baking dish. Dot with remaining butter. Cover loosely, and bake 15 minutes in the preheated oven. Remove cover, and continue baking about 8 minutes, until lightly browned.