If not already done, hard-boil the eggs. Once cooked, cool them in an ice bath, peel, and finely chop. Set aside.

In a large mixing bowl, combine the well-drained tuna, chopped hard-boiled eggs, mayonnaise, minced red onion, chopped celery, Dijon mustard, fresh parsley, salt, and black pepper. Mix gently until all ingredients are well combined. Taste and adjust seasoning as needed. Cover and refrigerate while you prepare the pastry.

Preheat your oven to 400°F (200°C). Line a baking sheet with parchment paper.

On a lightly floured surface, carefully unroll the thawed puff pastry sheet. Gently roll it out slightly with a rolling pin to a rough 10x12 inch (25x30 cm) rectangle.

Cut the pastry into 6 equal rectangles (approximately 5x4 inches or 12.5x10 cm each).

In a small bowl, whisk together the remaining large egg and 1 tablespoon of water to create an egg wash.

Place about 2-3 tablespoons of the tuna and egg filling onto one half of each pastry rectangle, leaving a small border around the edges.

Lightly brush the edges of the pastry rectangles with the egg wash. Carefully fold the other half of the pastry over the filling, aligning the edges to form a turnover. Press down firmly around the edges to seal.

Using a fork, crimp the sealed edges of each turnover to ensure a tight seal and create a decorative pattern.

Transfer the turnovers to the prepared baking sheet. Brush the tops of each turnover generously with the remaining egg wash. If desired, cut a small slit or two on top of each turnover to allow steam to escape during baking.

Bake for 18-20 minutes, or until the pastry is golden brown and puffed up. Baking time may vary depending on your oven.

Remove from the oven and let cool slightly on the baking sheet before serving. They are delicious warm or at room temperature.