All-purpose flour, for rolling
2 sheets frozen puff pastry (each about 8½ ounces), thawed
½ cup Dijon mustard
1 large egg
Poppy seeds, for topping (optional)
Position the racks to divide the oven into thirds and preheat the oven to 400 degrees F. Line two baking sheets with silicone baking mats or parchment paper. Have a ruler and a pizza cutter (or sharp knife) at hand.
Working with 1 sheet of pastry at a time, roll the dough out on a lightly floured surface until you have a rectangle that’s about 12 x 16 inches. If necessary, turn the dough so that a short side of the rectangle is closest to you. Measure the length so that you can find the middle, and spread ¼ cup of the mustard over the lower half of the dough, stopping about 1/8 inch from the side and bottom edges. Fold the top portion of the dough over the bottom and, using the pizza cutter (or knife), with your ruler as a guide, cut the pastry from top to bottom into strips about 1 inch wide (I actually use the width of the ruler itself as my guide), then cut the strips crosswise in half. (If you prefer, you can leave the strips long.)
Carefully transfer the batons to one of the baking sheets and chill or freeze them while you work on the second batch. (You can make all the strips to this point and freeze them on the baking sheets, then pack them airtight and keep them frozen for up to 2 months.)
Lightly beat the egg with a splash of cold water and brush just the tops of the strips with this glaze. If you’d like, sprinkle them with poppy seeds.
Bake the batons for 8 minutes. Rotate the sheets from front to back and top to bottom and bake for another 7 or 8 minutes, or until the strips are puffed and golden brown. Remove the baking sheets from the oven and let the batons rest for a couple of minutes before serving.
Unbaked batons can be kept in the freezer for up to 2 months and baked while still frozen. Brush them with the egg wash and sprinkle them with the poppy seeds, if using them, just before baking.
Tapenade Batons. Can be made with tapenade instead of mustard, Before folding over the puff pastry, sprinkle the tapenade with grated lemon zest and/or grated Parmesan; other good add-ins are teensy slivers of roasted peppers or sun-dried tomatoes, paper-thin slices of onion, and toasted sliced almonds.