Preheat your oven to 400°F. While it heats, prepare all your ingredients: trim the asparagus spears to about 5-6 inches, removing the woody bottom ends. In a small bowl, whisk together the beaten egg with 1 teaspoon of water to create an egg wash. In another small bowl, combine the poppy seeds, sesame seeds, and garlic powder together. This advance prep ensures everything moves smoothly when you start assembling.

Remove the puff pastry from the refrigerator and work quickly to keep it cold—this is crucial for proper puffing during baking. On a lightly floured surface, roll the puff pastry to about 1/4 inch thickness. Using a sharp knife, cut the pastry into 6 even squares, approximately 4x4 inches each. Keep the cut squares on a parchment-lined baking sheet as you work to prevent them from warming up.

For each pastry square, spread about 1/2 teaspoon of Dijon mustard lightly across the center. Layer one slice of ham, then arrange 3 trimmed asparagus spears diagonally across it, then top with 1.5 slices of cheese. I like to fold the cheese to fit the shape of the pastry so it stays contained during baking. Leave about a half-inch border around the edges uncovered to ensure the pastry seals properly.

Working with one pastry square at a time, fold two opposite corners up and over the filling so they meet in the center, then fold the other two opposite corners up and over to create an envelope shape. Using a fork, gently press and crimp the overlapping pastry edges together to seal, creating a decorative pattern. Make sure the seal is tight so the filling doesn't leak out during baking, which can prevent proper puffing.

Brush the top of each folded pastry packet generously with the egg wash from Step 1, which will create a golden-brown, glossy finish. Immediately sprinkle the seed and garlic powder mixture evenly over the egg wash while it's still wet so it adheres properly. I find tapping the mixture gently with your finger helps it stick better than just sprinkling.

Place the baking sheet with the pastry packets in the preheated 400°F oven and bake for 12-15 minutes, until the pastry is puffed and deeply golden brown on all sides. The pastry should rise about 1-2 inches above the baking sheet. Remove from the oven and let cool for 2-3 minutes before serving warm so the filling sets slightly and won't spill out.