Thaw the pastry sheet at room temperature for 40 minutes or until it's easy to handle.
Heat the oven to 400 degrees F. Line a baking sheet with parchment paper.
Unfold the pastry sheet on a lightly floured surface.
Cut the pastry sheet in half lengthwise into 2 (4 1/2 x 9-inch) rectangles.
Place the pastry rectangles onto the prepared baking sheet. Cover with another piece of parchment paper and top with another baking sheet.
Bake for 20 minutes or until the pastries are golden.
Remove the pastries to a wire rack and let cool completely.
Line a 9x5-inch loaf pan with plastic wrap.
Spread the lemon sherbet in the bottom of the loaf pan. Trim the edges of 1 pastry rectangle to fit the loaf pan and gently press onto the lemon sherbet. Repeat the layers with the raspberry sherbet and remaining pastry rectangle. Cover the loaf pan and freeze for 4 hours or overnight.
Place the raspberries and sugar into a blender or food processor. If using fresh berries, let the mixture stand for 20 minutes. Cover and blend until the mixture is smooth. Press the raspberry mixture through a sieve into a medium bowl.
Invert the loaf pan onto a serving plate.
Cut the loaf into 8 slices.
Drizzle the slices with the raspberry sauce and top with the raspberries, if desired.
