Yes. Egg casserole leftovers will keep three to four days in the fridge. But you can also cut it into portion-size pieces and flash freeze the cooled squares. Once they are frozen, put the one serving pieces into a zip-top freezer bag, removing as much air as possible. Freeze for up to three months for a quick and easy breakfast option. To cook, thaw in the fridge overnight and then reheat in the air fryer or microwave.