Store the leftovers in an airtight container in the refrigerator. The potatoes will last for several days when stored properly. When reheating the potatoes and onions, you can microwave them until warm or cook over medium-high heat in a skillet with a little bit of butter or oil, on the stovetop. I prefer to heat them in the skillet as they tend to get a bit soggy when reheated in the microwave.