Roll out puff pastry to a 3/16-inch thickness.
Cut a 12 X 6-inch rectangle for base of tart, and place on a dampened baking sheet.
Cut 4 strips of pastry 1 1/2 inches wide to make sides of tart €”2 strips will be 12 inches long and 2 will be 6 inches long.
Dampen edges of pastry base, and lay strips in place.
Pinch slightly to make them high rather than flat.
With a fork, prick base of the tart shell at 1-inch intervals.
Freeze tart shell on baking sheet.
Preheat oven to 375 °F.
Bake frozen tart shell for 35 to 40 minutes, or until golden brown.
If base rises in the center during baking, prick again with a fork.
Cool baked shell on baking sheet on a wire rack.
When ready to assemble tart, spread Pastry Cream on bottom of shell.
Make alternate diagonal stripes about 2 inches wide of blackberries and raspberries.
Spoon jelly into a small saucepan, melt over low heat, and brush liberally over berries.