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1
On a Silpat or plastic wrap covered large cutting board, roll out one sheet of puff pastry at a time into a 9 x 11 rectangle. (The exact measurement isn't terribly important -- you just want to thin it out a bit and you want to keep it in a rectangular shape.)
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2
Orient the puff pastry horizontally so that the short sides of the rectangle are by your right and left hands. (This will make the rolling up easier.) Using half of the mustard, spread a thin coating all over the puff pastry. Sprinkle evenly with half of the cheese and half of the prosciutto.
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3
Fold in the short sides so halfway towards the center. Then, fold in the short sides again so that they meet and almost touch in the center. Finally, fold one side over the other side so that you have one long rectangle in front of you (that'll be oriented vertically).
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4
Place folded puff pastry on a Silpat or parchment-lined baking sheet, cover lightly with plastic wrap and refrigerate. Repeat process with remaining sheet of puff pastry, mustard, cheese and prosciutto. Chill both for at least an hour or up to a day. Or, wrap well in plastic wrap and freeze.
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5
When ready to bake the palmiers, preheat the oven to 425. Lay a prepared sheet of pastry on a cutting board and slice, horizontally, into 1/4 thick slices. Repeat with other prepared sheet of pastry. (No need to thaw if you've frozen the palmiers first.)
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6
Lay each palmier (slice of the prepared puff pastry) with the ingredients facing up on back on the parchment- or Silpat-lined baking sheet.
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7
Beat egg with a tsp of water and brush the palmiers with the egg wash. (Feel free to skip this step if you want, but it does give the finished palmiers a nice appearance.)
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8
Bake for approximately 12 minutes until they've turned a nice browned color. (If you've frozen them first, you might need another minute in the oven, but you may not.)
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9
Enjoy either hot, warm or at room temperature. Or, if you prefer, put the palmiers in the freezer on a baking sheet and, once frozen, transfer them either to a freezer bag or another container.
