Preheat oven to 400 degrees and line two baking sheets with parchment paper.
On a lightly floured surface, roll out each sheet of puff pastry into a thin rectangle, then cut each into 6 squares (yielding 12 squares total).
In a large bowl, toss together asparagus, olive oil, salt, pepper, onion powder and garlic powder.
In a medium bowl, combine Gruyere and Parmesan.
In a small bowl, whisk egg yolk and water.
To assemble, fold a piece of prosciutto in half and place on a puff pastry square. Lay three asparagus spears on top, then top with about 2 tablespoons cheese mixture. Wrap one corner of puff pastry over the filling, brush corner with egg wash, fold other corner on top and press to seal. Repeat with remaining pastry and filling.
Brush tops with egg wash, and sprinkle with sesame seeds, if desired. Place in oven and bake for about 15 minutes, until puff pastry is golden brown. Serve and enjoy!