1. Heat the oven to 425°F.
2. Unfold the pastry sheet on a lightly floured surface. Cut the pastry sheet into a 9-inch circle. Place the pastry circle onto a baking sheet.
3. Bake for 10 minutes or until the pastry is golden brown. Remove the pastry from the baking sheet and let cool completely on a wire rack.
4. Cut the bananas in half lengthwise then crosswise into slices.
5. Heat 1/2 cup brown sugar, the rum, butter, lemon juice and cinnamon in a 10-inch skillet over medium heat to a boil. Cook and stir for 2 minutes or until the mixture is thickened. Add the bananas and toss to coat.
6. Stir the sour cream and remaining brown sugar in a small bowl. Spoon the banana mixture onto the pastry. Top with the sour cream mixture.