Preheat the oven to 375 degrees Fahrenheit and line 2 large baking sheets with parchment paper.
In a large bowl, combine the 1 pound of ground pork, 1 egg, ¼ cup finely minced onion, 2 cloves of minced garlic, 1 tablespoon Dijon mustard, 1 teaspoon dried thyme, 1 teaspoon dried rosemary, 1 teaspoon dried sage, ½ teaspoon salt, ½ teaspoon pepper, and ⅛ teaspoon nutmeg. Stir until evenly mixed.
Add 1-2 ice cubes and 2-3 tablespoons of cold water to the sausage mixture. Let the ice melt slightly, then stir. The mixture should be loose enough to pipe from a piping bag.
Spoon the sausage mixture into a large piping bag and snip a 1/2 inch opening from the tip. Set aside.
In a small bowl, whisk the remaining egg with a splash of water to make an egg wash. Set aside.
Lightly flour your work surface. Unfold one sheet of puff pastry and gently roll it out with a rolling pin to smooth the seams. Cut the puff pastry in half lengthwise so you have two smaller sheets. (please see the step-by-step images in the full post for visual guidance if needed)
Pipe a 1-inch thick line of sausage mixture along the long edge of each strip.
Brush the opposite long edge with egg wash, then roll the pastry up tightly around the filling, ending with the egg washed edge on the outside to seal. Repeat with the second sheet of puff pastry so you have four long filled logs total.
Refrigerate for 30-60 minutes (time allowing).
Slice each long roll into 4-5 equal pieces and place them seam side down on the prepared baking sheet. Score the tops of each roll to allow steam to vent, then brush lightly with more egg wash.
Bake for 20 to 25 minutes, or until the pastry is golden brown and the sausage is cooked through (160℉).
Let the baked sausage rolls cool on the pan for about 5 minutes, then transfer to a wire rack.
Serve warm with ketchup, mustard, and a sprinkle of freshly minced parsley, if desired.