Remove the puff pastry from the package and unroll. Cut out a 20 cm circle. Cut the remaining pastry into strips and form into an edge.
Heat about 5 tablespoons of milk in a saucepan. Stir in the pudding powder, remaining milk and sugar. Mix until smooth. Bring to a boil, stirring constantly. Puree 200 grams (approximately 7 ounces) of blueberries and mix into the pudding. Separate the egg. Fold in the egg white.
Sprinkle the puff pastry with breadcrumbs and spread the blueberry pudding on top. Smooth and cover with the remaining blueberries. Mix the egg yolk with a little water and brush over the pastry. Bake for about 30 minutes at 200°C (approximately 400°F). Remove from the oven, let cool and serve.