Place your pastry on an ungreased cookie sheet and bake according to package instructions. If you want a more golden, shiny top brush with egg wash and sprinkle with sugar.
In a small sauce pan add 2 cups of your blackberries with 2 tsp. sugar and your lemon juice. Cook until boiling then lower the heat to a simmer. Cook the blueberries until they are completely broken down and you have a smooth sauce. If the sauce is thick, add a few drops of water until it is the right consistency.
While your sauce is cooking add 3 teaspoons of sugar to a small bowl and add the remaining blueberries, toss to coat. Cover bowl and let them sit while the sauce finishes cooking.
Mascarpone Whipped Cream
Add your whipping cream and cheese into a small bowl. Using a mixer beat on high for 3 to 5 minutes or until soft peaks form.
How to assemble
Spoon a little of the blueberry sauce on your puff pastry.