Preheat oven to 400 F. Cook the bacon until crisp in a large saucepan over medium high heat, drain and reserve drippings, and cut into 1 inch pieces. Place the bacon and its drippings in a large bowl. Add the turnips, potatoes, cream, rosemary, salt, pepper and garlic and toss to combine. Transfer to a large, shallow baking dish, about 13 by 9 by 2 inches, dot the top with butter and cover tightly with foil. Bake for 50 minutes, remove the foil and continue to bake until vegetables are tender and the top browns, about 25 minutes. Serve hot.