1. Preheat oven to 425F. Butter a 6 cup shallow gratin dish.
2. Peel turnips and sweet potatoes. Slice thin (with a chef knife or on a mandolin). Layer half of turnip and sweet potatoe slices in dish. Top with garlic powder, salt , pepper and half of cheese. Top with remaining turnips and sweet potatoes, and cheese. Pour cream carefully over mixture, getting it down into the crevices.
3. Cover and bake 45 minutes. Uncover and bake 15-20 minutes longer or until mixture is bubbly and potatoes are tender and browned. Serves 8.