Preheat oven to 400°F. Line a large baking sheet with parchment paper.
Place the asparagus in a large bowl. Drizzle with olive oil, sprinkle with black pepper, and toss gently to coat. Set aside.
Lightly flour a clean work surface. Unfold the thawed puff pastry and gently roll it into approximately a 12-inch square.
Cut the puff pastry into 8 equal squares.
Place one pastry square on the work surface with a corner pointing toward you. Add about 2 tablespoons of shredded Gruyère cheese diagonally across the center.
Top the cheese with a few slices of pancetta and 3 asparagus spears, positioning the asparagus so the tips extend beyond the pastry.
Fold one corner of the puff pastry over the filling. Brush the top lightly with water, then fold the opposite corner over it and gently press to seal. Transfer seam-side up to the prepared baking sheet.
Repeat with the remaining puff pastry, Gruyère, pancetta, and asparagus.
In a small bowl, whisk together the egg and 1 tablespoon water to make an egg wash.
Brush the exposed puff pastry generously with egg wash.
Bake for 18–22 minutes, or until the puff pastry is deeply golden and puffed, the cheese is melted, and the asparagus is tender.
Let cool on the baking sheet for about 5 minutes before serving. Serve warm.