To store your potato gratin, let it cool to room temperature, then cover it tightly with plastic wrap or aluminum foil and pop it in the fridge. This dish stays cozy and tasty for up to 5 days. When you're ready to reheat, warm it in the oven at 350°F until it's heated through and the top gets crispy again, usually about 20-25 minutes. Microwave touch-ups are quick but you might sacrifice some of that awesome crunch.