1In 1 1/2-quart saucepan, stir sugar, cornstarch and water until smooth. Add 1 cup of the blueberries. Heat to boiling; reduce heat. Simmer uncovered 5 minutes, stirring occasionally. Stir in lemon juice. Cool 15 minutes. Stir in remaining blueberries and the raspberries. Cover and refrigerate at least 1 hour but no longer than 24 hours.
2Heat oven to 400ºF. Let frozen pastry sheet stand at room temperature 15 minutes. Cut pastry into 8 pieces. Place pieces on ungreased cookie sheet. Bake about 15 minutes or until puffed and golden brown. Remove from cookie sheet to wire rack; cool slightly. Split into 2 layers while warm.
3To serve, place puff pastry on individual serving plates. Fill and top with sauce. Top with small scoop of sherbet.