Preheat oven to 400 degrees F. Unfold puff pastry dough and Roll into a 14×10″ rectangle on a lightly floured surface. Move the puff pastry to a parchment-lined baking sheet. Score the dough 1″ from the edge all around the rectangle with a knife. Dock the inside rectangle by poking it lightly with a fork in approximately 1/2″ spaces. Bake for 15 minutes.
While the pastry is baking, prepare your asparagus spears by rinsing and removing the woody ends. Set aside. In a small bowl, mix together sour cream, egg yolks and Dijon mustard. Stir in 1/4 cup of the Gruyère.
Remove the puff pastry from the oven and spread the sour cream mixture evenly over the inside rectangle. Top with the remaining Gruyère cheese and layer with the asparagus spears. Return to the oven and bake for 15 additional minutes until cheese begins to brown lightly and asparagus is cooked.
Remove from oven and allow to cool slightly. Slice using a pizza cutter and serve.
Adapted from goodlifeeats.com