In a muffin tin, cover the bottom of five cavities with 1 oat bar each and press into an even layer. Set aside.
In a bowl, combine the brown sugar and cornstarch. Add the apples and lemon juice. Mix well.
In a pot, bring the sugar and water to a boil. Cook without stirring until golden caramel forms. Add the butter and mix until melted. Watch out for splattering. Add the apple mixture and cook for 10 to 15 minutes, stirring often, until the apples are tender and the liquid has evaporated.
Divide the apple mixture among the crusts in the muffin tin, pressing down lightly. Cover and refrigerate for 2 hours.
Using a small non-metallic knife or spatula, unmould the tartlets.