Normally, you'll just store any leftovers in the fridge. But if you wish to freeze it, you can. While I recommend covering it well and freezing it after it's already been baked, you can also freeze it before baking (though I'd add the topping later before actually baking it). Keep in mind it'll keep longer in the freezer if it's already been cooked. Just be sure you have plenty of time for it to thaw and re-heat when taking it out of the freezer. Allow time for it to thaw in the fridge before cooking or re-heating.