Pre heat oven to 400 degrees F. Line a baking sheet with parchment paper.
Roll out the thawed puff pastry onto the parchment paper.
Use a fork pierce or dock the pastry in the middle (except for the edges of the puff pastry, leave an 1 inch border around the edges that is not docked).
Bake puff pastry in the oven for 15 minutes. Remove from oven.
Using the back of the fork, light press down in the middle of the puff pastry to deflate. Do not press near the border. Only press down where the docking was done on the pastry.
Brush the balsmic vinegar on the puff pasty (in the middle, not the border). Sprinkled the Gruyere cheese onto the pastry (not the edges). Trim the asparagus spears ends to match the length of the pastry. Line the asparagus spears in the middle to form the pastry.
Return to oven. Cook for about 15-20 minutes or until the pastry becomes golden brown. Keep an eye on the tart while cooking.
Remove from oven and let cool for a couple minutes. Slice and serve.