On a piece of parchment paper, roll out the defrosted puff pastry sheet into about 10x10 inch square.
Spread mustard on the puff pastry, leaving a 1/2 inch border.
Layer the prosciutto slices side-by-side on top of the mustard, try not to overlap them too much. Sprinkle the cheese evenly across the prosciutto slices and then sprinkle over the freshly minced sage.
Start from the bottom and tightly / gently roll up into a log. Wrap the log in the parchment paper and place in the freezer for 10 minutes while you preheat the oven to 400F.
Line a large baking sheet with parchment paper and set aside.
After 10 minutes, remove the puff pastry log from the freezer and cut into 1/2 inch slices using a sharp or serrated knife. You’ll end up with about 18-22 pinwheels.
Place them on the baking sheet, making sure to leave space between each pinwheel and brush the top and sides of each with egg wash (1 egg whisked with 1 tbsp water) if desired.
Bake for 22-28 minutes until golden brown. Leave on the baking sheet to cool.
Allow to cool for 5-10 minutes and then serve on a plate, sprinkling on more fresh minced sage leaves if desired.