Adapted from Mustard Batons by Dorie Greensspan
~from Savoring Time in the Kitchen
2 sheets frozen puff pastry, thawed
All purpose flour, for rolling out puff pastry
½ cup Dijon mustard
1 large egg
Coarsely ground pepper
Sea Salt
A ruler
Pizza or pastry cutter
Place the racks inside the oven so that the oven is divided into thirds. Preheat oven to 400F. Place a silicone baking mat or parchment paper on two large baking sheets.
Take one pastry sheet out of the package and keep the other refrigerated until ready to use. On a lightly floured surface, roll out the dough until it measures 12 x 16 inches, approximately, with the shortest end facing you. Find the middle and mark it lightly. On the lowest half, spread about ¼ cup of mustard, stopping about 1/8 inch from the side and bottom edges. Fold the top of the dough over the bottom. Mark the pastry every ¾ inch or so, then cut into strips using a pizza/pastry cutter or sharp knife.
Carefully, transfer the strips to one of the lined baking sheets (keeping about 2 inches between them since they will puff up) and chill or freeze while you work on the second sheet of puff pastry. At this point, you can freeze them on the sheets and then wrap air-tight until ready to use for up to 2 months.
When ready to bake, beat the egg with a little cold water and brush the tops of the strips. Sprinkle them with coarsely ground pepper and sea salt (Dorie suggests poppy or sesame seeds).
Bake for 8 minutes, rotate and transfer the sheets, then bake for another 7 to 8 minutes until golden brown. Cool slightly before serving.