Step 1

Preheat oven to 400 degrees F.  Line a baking sheet with parchment paper.

Step 2

Use a rolling pin to gently roll out puff pastry to an 11"x14" rectangle, then transfer to the parchment lined baking sheet.  Dock (a fancy baking term meaning to prick all over) the puff pastry with a fork, then brush all over with the mustard.

Step 3

Sprinkle gruyere over the puff pastry, leaving a 1" border on all sides. Arrange asparagus spears to be parallel with each other (or get fancy and arrange them on a diagonal!).

Step 4

Sprinkle with the Parmesan cheese.  Bake for 20-25 minutes or until the crust is golden brown and puffy.  Slice into squares a serve.  If desired, you can also drizzle a balsamic glaze (thicker and sweeter than balsamic vinegar) on top.