Preheat oven to 400 degrees.

On a silicone baking mat or parchment paper, roll the puff pastry into a rectangle, approximately 16 x 10 inches. Lightly score the edges to make a "crust". On the interior of the puff pastry, poke holes with a fork.

Transfer the pastry (on the silicone mat or parchment paper) onto a sheet pan. Bake for 15 minutes.

Remove from oven and using a spatula lightly flatten the middle of the pastry that may have puffed up during baking. Allow to cool a bit.

Sprinkle pastry crust with shredded cheese and arrange asparagus spears evenly atop the cheese. Lightly drizzle the asparagus spears with olive oil and sprinkle with salt.

Bake for 18 - 20 minutes, until cheese is melted and crust is nicely browned.

Remove from oven and allow to cool about 5 minutes. Then drizzle evenly with balsamic vinegar reduction. Serve warm or at room temperature.