Prep: 1. Peel and cut sweet potatoes. 2. Separate egg whites from yolk. Make: 1. Preheat oven to 375. 2. Butter an 8-inch square pan baking pan. 3. Arrange sweet potato slices in layers, sprinkling with salt, pepper, and thyme as you go. 4. Cover with foil and bake until tender, 45-90 minutes. 5. Meanwhile, over high heat, bring 1 cup water to boil in medium pot. Reduce to simmer. 6. Put egg whites and cream of tartar in a rimmed metal bowl just big enough to rest on rim of pot. 7. Set bowl over pot and whisk egg whites constantly until hot but not cooking, 3-5 minutes. 8. Remove from heat and beat with a mixer until firm peaks form. Sprinkle in sugar and vanilla and beat into stiff, shiny peaks. 9. Spread over cooked sweet potatoes and broil 6 inches from heat until browned. Serve.
Can be made a day ahead! Save step 9 until you are just about ready to serve. Bring to room temperature and reheat sweet potatoes in a oven heated to 375, about 15 minutes.