Preheat oven to 425 degrees F. Line two baking sheets with parchment paper.
In a saucepan combine blueberries, sugar, cinnamon, nutmeg, lemon juice, salt, and butter. Cook over medium-low heat until berries soften (5 minutes). Stir in flour and cook until thickened (about 2 minutes). Remove from heat and set aside to cool.
Lightly flour a work surface. Unroll puff pastry and cut into thirds along the folds.
Spoon cooled filling down the center of each piece of puff pastry. Bring edges together and pinch to close. Don't overfill or they will break open during baking. Slice into 1-1/2 inch lengths. Place rolls on baking sheets.
Mix egg (or egg white) with a small amount of water. Whip with a fork until foamy. Brush the top of each roll with the egg mixture. Prick with a knife or fork to vent (optional).
Place on the parchment paper lined baking sheet and bake 15 to 20 minutes or until golden brown. Dust with powdered sugar. Allow to cool completely on the baking sheet.
This recipe was calculated using the exact brands and measurements I used to make this recipe. If you are following a strict diet please note changing anything will cause the nutritional info to change. My calculations are intended as a guide only.