Preheat oven to 375 degrees. Place tart shells on a baking sheet.Combine lemon juice and apples in a mixing bowl. In a cast iron or heavy skillet, heat butter until frothy.Add apples and toss to cover with butter. Cook until apples are tender. Add brown sugar and cook for 3 or 4 minutes. .Fill tart shells with the apple mixture and top with caramel topping. Dollop with a spoon of Cool Whip.