Yes! To freeze a whole unbaked onion tart, place it on a baking sheet and freeze it until firm. Then, wrap it with a layer of heavy duty foil and place it in a zippered, freezer-safe plastic storage bag and freeze for up to 1 month. Unwrap and bake from frozen, adding 15 to 20 minutes extra time. To freeze a baked tart or individual slices, wrap and freeze the completely cooled tart as directed above. To reheat, unwrap and bake the frozen tart in a preheated 350 degree F oven for about 25 minutes, or until heated through.