My mother loved gratins, and leftover meat and cold cuts always found their way into our pasta, usually along with zucchini or peas. I have made gratins for my family through the years. The gratin can be made ahead, sprinkled with Parmesan, and kept, covered, at room temperature for a few hours or refrigerated for up to a day before it is finished in the oven. If refrigerated, bring back to room temperature and bake on a cookie sheet for about 30 minutes in a 425-degree oven until heated through and lightly browned on top. Do not try to keep the cooked gratin in a warm oven for more than 10 to 15 minutes, or the pasta will swell up in the liquid and become gooey.