Unfold one puff pastry sheet onto your work surface. Use the parchment paper to line the baking tray. Cut pastry into 4, 5×5-inch squares.
Place a ¼ cup of grated cheese diagonally onto each square. Arrange 3 asparagus spears and 2 slices of prosciutto on top of cheese.
Fold the two opposite corners of pastry towards the center and overlap to seal. Transfer pastry to baking tray. Sprinkle with sea salt. Repeat with second pastry sheet.
Bake for 18-20 minutes until pastry is golden brown. Serve warm with a side of your favourite mustard and enjoy!