Preheat your oven to 400°F (205°C) and line a large baking sheet with parchment paper. Snap off the woody ends of the asparagus spears. In a medium bowl, toss the trimmed spears thoroughly with the extra virgin olive oil, kosher salt, cracked black pepper, and optional garlic powder until evenly coated.

Roll out your cold, thawed puff pastry sheet onto a very lightly floured surface into a clean rectangle. Using a sharp knife or pizza cutter, slice the sheet evenly into 6 equal squares.

Turn a pastry square so it sits in a diamond orientation in front of you. Fold one slice of beef prosciutto in half so it fits neatly inside, laying it horizontally across the center. Place 3 to 4 seasoned asparagus spears diagonally across the meat, then top the center with roughly 2 tablespoons of shredded Gruyère cheese.

Lift the two opposite side corners of the pastry square and wrap them tightly over the top of the filling to meet in the center. Overlap the edges and press down firmly to seal, using a tiny dab of water or egg wash on your fingertips to glue the dough securely together. Transfer the completed bundle to your prepared baking sheet and repeat for the remaining squares.

Brush all exposed puff pastry surfaces generously with the beaten egg wash mixture. Sprinkle a tiny bit of extra black pepper or flaky sea salt over the top if desired. Bake for 15 to 18 minutes, or until the pastry is puffed and deeply golden brown, and the cheese is melted and bubbly. Let cool on the pan for 3 to 5 minutes before serving warm.