Preheat oven to 400°F.
Place half the bag of frozen blueberries, sugar, and lemon juice in a heavy-bottomed sauce pan over medium-high heat. Cook for 5-6 minutes until the berries burst and the juice comes out. Remove about 1/4 cup of berry liquid and reserve for the strip steak pan sauce.
Strain remaining liquid into a blender, discarding the burst berries. Add the remaining half of the frozen blueberries to blender and blend until smooth. Add heavy cream, blending until incorporated. Store in refrigerator until ready to eat.
Cut puff pastry into one-inch squares. Line a sheet pan with parchment paper and place squares on the pan about one inch apart. Bake until golden brown, about 10-15 minutes. Toss croutons in sugar while hot, then let cool.