To reheat an entire refrigerated onion tart, first let it rest on the kitchen counter for about 30 minutes to take the chill off. Meanwhile, preheat your oven to 350 degrees F. Place the tart on a baking sheet, cover it with aluminum foil, and bake for 15 to 20 minutes or until the internal temperature reads 165 degrees F. You can reheat individual slices in the microwave for 20 second increments until warmed through.