Preheat oven to 425 degrees. Line 2 baking sheets with parchment paper and set aside.
Beat egg in a large bowl until frothy. Add the sausage, breadcrumbs, garlic, onion, parsley, and mustard. Mix until thoroughly combined.
Unwrap first sheet of puff pastry and place on a floured surface.
Cut along the fold lines to create three strips of dough.
Divide sausage mixture into 6 equal parts.
Make each section of sausage into a roll and place down the center of puff pastry strips.
Fold pastry over and press seam to seal.
Cut each roll into 6 equal pieces (about 2 inches).
Place on baking sheet and brush with egg wash.
Bake for 15 to 20 minutes, until puffed and golden brown.
Follow the same steps with the remaining sausage and puff pastry.
Serve sausage rolls while warm.
Store leftover rolls in the refrigerator. Reheat in the microwave or heated oven.