2. Using the back of a spoon or an offset spatula, spread the mustard over the pastry. Distribute the Gruyère and Parmesan evenly over the surface. Arrange the ham in a single even layer, tearing or cutting pieces to fit. Lay a piece of parchment paper or waxed paper on top and gently compress the layers with the rolling pin. Peel off the paper without disturbing the ham.
3. Cut the rectangle in half widthwise to make two 10-by-7-inch bands. With your fingers, gently roll one short edge of one of the bands into the center and then roll the opposite edge in so the two rolls meet in the middle and resemble a double scroll. Press lightly so the rolls stick together (spread a few drops of water where the two rolls meet to help them stick, if you need to). Repeat with the second band. Wrap the rolls in plastic and chill until they firm up, at least an hour in the refrigerator or 30 minutes in the freezer.
4. When you're ready to bake, heat the oven to 425°F and line two baking sheets with parchment paper or a silicone baking mat (or use nonstick baking sheets). With a very sharp knife, slice each roll into 12 pieces, arrange them on a sheet at least 1 inch apart, and bake until the pastry is nicely browned and flaky (break one apart to be sure it's not still doughy in the center), and the cheese is melted but not burned, 10 to 12 minutes. If your oven doesn't heat evenly, swap the pans' positions halfway through cooking. Transfer the palmiers to a cooling rack. Serve just slightly warm or within the hour, if possible.
Reprinted from Puff: 50 Flaky, Crunchy, Delicious Appetizers, Entrees, and Desserts Made with Puff Pastry by Martha Holmberg (Chronicle Books, 2008). Copyright 2008 by Martha Holmberg.
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