Umami-rich miso adds savoriness to this smooth, sweet potato gratin spiked with fresh ginger and soy sauce. We liked mellow white miso best, which has a milder flavor than yellow or red miso. For a crispy topping, we toasted panko breadcrumbs in butter, then mixed them with sesame seeds, brown sugar and white pepper. To make holiday-dinner prep more convenient, the gratin can be made up to a day ahead. Roast the sweet potatoes, cool them enough to peel, then make the sweet potato mixture as instructed. Transfer to a baking dish, cover with foil and refrigerate overnight. The next day, bring to room temp for 1 hour, then bake at 400°F for 50 minutes. In the meantime, make the topping. Once the sweet potatoes are warmed through, sprinkle with the topping and bake as instructed for 20 minutes.