Using a 10-centimeter biscuit cutter, cut four rounds from the pastry sheet, discarding the trimmings. Using a fork, prick each pastry round about twelve times to prevent the dough from rising too much in the oven. Following package directions, bake the pastry rounds on a baking sheet until golden brown. Transfer to a wire rack to cool.
In a small saucepan over medium heat on the stove, combine the sauce ingredients, stirring constantly until the sugar is dissolved and the butter is melted, 4 to 5 minutes. Remove from the heat and set aside.
In a large bowl toss the apple slices with the melted butter to coat well.
Prepare the grill for direct cooking over medium heat (180° to 230°C) and preheat a perforated grill pan.
Put the apple slices in a single layer in the grill pan and grill over direct medium heat, with the lid closed, until they are well browned and tender, 8 to 10 minutes, turning once or twice. Transfer the apple slices to a bowl. Reheat the sauce over low heat, if necessary, and spoon some of the sauce over the apple slices, gently tossing to coat evenly.
To assemble, place each pastry round on a dessert plate. Arrange the apple slices on top of the pastry rounds. Spoon the remaining sauce over the apples, allowing it to run down onto the plates. Sprinkle with a little coarse sea salt, if desired. Finish each with a tablespoon of crème fraîche or whipped cream.