You can make potatoes au gratin in advance and then reheat them on the stove, in the oven or in the microwave, making it easier to plan and cook for a large dinner. Store your potatoes au gratin in a microwave or heat-safe dish for easy reheating, and follow the right timing to prevent your potatoes from drying out or burning.
As a casserole dish, potatoes au gratin need to be heated to 165 degrees Fahrenheit to minimize the risk of food poisoning. While you can see them bubbling and browning in the oven, only a quick-read thermometer will tell you the dish is heated to the correct temperature. If you're reheating frozen au gratin, thaw it completely in your refrigerator prior to reheating. However, this can take one to four days, so if you don't have time to thaw in the refrigerator, defrost the dish in the microwave. Cover it with a layer of plastic wrap, leaving one corner open for venting. Use the defrost setting on the microwave, cooking at two-minute intervals until the gratin is completely thawed.
Reheating on the Stove or in the Oven
Cover your defrosted potatoes au gratin with a layer of foil or an oven-safe lid. Preheat your oven to 350 F, and place the dish on the middle rack. Cook for 25 to 30 minutes, or until the internal temperature reads 165 F with a quick-read thermometer. Remove the foil covering for the final five to 10 minutes to let the top brown further and crisp up.
On the stove, cover your potatoes with a lid, and heat gently on medium-low heat for 20 minutes. Be careful to not scald the bottom of the potatoes. If you browned your potatoes before cooling and storing, don't stir. If you didn't brown them, a quick stir can help prevent sticking.
Using the Microwave
Cover your potatoes au gratin with a piece of microwave-safe plastic wrap, leaving one corner open for venting. Microwave on high for three minutes, stopping for 30 seconds, and then heating again for another three minutes. Heat until the internal temperature reads 165 F. Keep the reheated potatoes au gratin covered so they stay warm until served. Heating in the microwave won't brown the potatoes, so brown prior to cooling and storing.
Serving and Storage
Serve reheated au gratin as you would when it's freshly cooked. The creaminess makes it a good cool-weather dish, and its richness goes well with roasted meats, such as turkey or chicken. Any extra au gratin can be stored in the fridge or the freezer; just cover so it's airtight, using plastic wrap or a sealed storage container to help keep out potentially harmful bacteria.