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1
Combine the tomato paste and water in a small bowl; set aside.
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2
Generously dust a work surface with flour.
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3
Place the thawed pastry on the prepared work surface, dust the pastry lightly with flour, and, using a sharp knife or pizza cutter, trim it into a 10-by-13-inch rectangle.
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4
Using a pastry brush, coat the surface of the pastry evenly with the tomato-water mixture, using all of it.
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5
Arrange the prosciutto slices in a single layer over the tomato paste, overlapping slightly.
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6
Sprinkle the cheese evenly over the prosciutto.
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7
Fold each of the long sides of the pastry in toward the center until they meet.
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8
Fold again along the center seam, as if closing a book.
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9
Wrap the roll in plastic wrap and refrigerate until firm, about 30 minutes
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10
Meanwhile, heat the oven to 400 degrees F and arrange the racks to divide the oven into thirds.
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11
Line 2 baking sheets with parchment paper and set aside.
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12
When the pastry roll is ready, remove it from the refrigerator and discard the plastic wrap.
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13
Using a serrated knife, trim about 1/2 inch off each end.
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14
Cut the roll crosswise into 1/2-inch slices.
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15
Arrange the slices on the prepared baking sheets about 1 inch apart.
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16
You should have about 12 palmiers on each sheet.
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17
Bake until the palmiers are lightly browned, about 8 to 10 minutes.
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18
Remove the baking sheets from the oven and flip the palmiers over with a flat spatula.
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19
Return to the oven and bake until deep golden brown, about 8 to 10 minutes more.
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20
Remove the palmiers to a wire rack to cool completely.
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21
If not serving right away, store at room temperature in a container with a tightfitting lid for up to 6 hours.
