Preheat oven to 400 degrees Fahrenheit.

Line two baking sheets with parchment paper or baking mats.

Lightly flour a surface.

Roll out one sheet of puff pastry into a rectangle, approximately 12 by 16 inches.

Position the pastry rectangle with a short side facing you.

Spread half of the Dijon mustard over the lower half of the pastry, leaving a 1/8-inch margin around the edges.

Fold the top half of the pastry over the mustard-covered lower half.

Using a sharp knife and a straight edge, cut the folded pastry into 1-inch-wide strips from top to bottom.

Transfer the strips (batons) to the prepared baking sheets, leaving a small space between them.

Refrigerate the baking sheets with the batons until ready to bake.

Repeat the rolling, mustard spreading, folding, and cutting process with the remaining pastry and mustard.

In a small bowl, beat the egg with a splash of cold water to create an egg wash.

Brush the egg wash lightly onto the batons on both baking sheets.

Sprinkle the batons with poppy or sesame seeds (optional).

Bake for about 15 minutes, rotating the pans halfway through after 8 minutes.

Continue baking until the batons are puffed and golden brown.

Let the batons rest for a few minutes before serving.