Preheat the oven to 425°F / 218°C. If using frozen puff pastry, thaw per packaging instructions. Use a rolling pin to gently flatten the sheets if necessary.

Cut each sheet into 4 equal squares to have 8 squares total, approximately 5 inches on each side. Brush each square with egg wash, fold into a triangle, and seal the edges. Brush the tops with more egg wash and sprinkle with coarse sugar if desired.

Bake for 22-25 minutes until deep golden brown. Set aside to cool completely.

While baking, prepare the jam by heating the frozen blueberries, blackberries, raspberries, sugar, and lemon juice in a pot over medium heat. Bring to a simmer and cook for 15-20 minutes until thickened. Set aside to cool.

For the cream filling, use a hand mixer to beat the heavy whipping cream, powdered sugar, and vanilla extract until stiff peaks form.

In a separate bowl, beat the cream cheese and sour cream until fluffy. Fold in the whipped cream in thirds until fully combined.

Cut each croissant in half without cutting through. Spread jam on both sides and fill generously with cream filling. Dust the tops with powdered sugar.