MAKE THE CRUST: In a medium bowl, combine graham cracker crumbs, melted butter, and granulated sugar until the texture resembles wet sand. Press the mixture firmly into the bottom of a 9x13-inch baking dish using the back of a spoon or measuring cup. Place in the freezer for 15 minutes to set.

PREPARE THE FILLING: In a large bowl, beat softened cream cheese and powdered sugar together until smooth and fluffy. Add vanilla extract and lemon juice, mixing until fully combined. In a separate bowl, whip the heavy cream until stiff peaks form, then gently fold it into the cream cheese mixture until light and airy. Spread the filling evenly over the chilled crust.

DECORATE THE FLAG: Arrange frozen blueberries in the upper left corner of the cheesecake to represent the stars on the flag. Create red stripes by lining up sliced strawberries across the cake. Leave gaps between the strawberry rows to form white stripes from the filling. Optionally, include frozen raspberries or cherries for extra texture, and lightly dust the top with powdered sugar if desired.

FREEZE AND SERVE: Cover the cheesecake with plastic wrap or foil and freeze for at least 4 hours, or overnight for firmer texture. Before serving, allow it to sit at room temperature for 10 to 15 minutes to slightly soften. Cut into squares and serve cold.