• Adjust oven rack to the lower-middle position and preheat to 375°F (190°C). Line a rimmed baking sheet with parchment paper. If making individual tarts, cut the puff pastry as desired—I prefer using half of a pre-cut sheet and cutting it into two sections. Transfer pastry to the prepared baking sheet.

Make the Filling:

• Microwave cream cheese in a large bowl until softened, about 20-30 seconds.
• Whisk in ¼ cup Parmesan, olive oil, Dijon mustard, ½ teaspoon rosemary, pepper, and egg yolk until smooth.
• Stir in potato slices and shallot, making sure they’re well coated.

Assemble the Potato Tart:

• Spoon the potato mixture onto the center of the puff pastry, spreading into an even layer while leaving a 2 -inch border on all sides. Sprinkle 3 tablespoons Parmesan and remaining ¼ teaspoon rosemary over the filling.
• Fold one long side of the dough over the filling, about 1½ inches. Repeat with the opposite long side, then fold in the short sides, overlapping corners to secure. Lightly beat the egg white and brush over the folded crust (you won’t need it all). Sprinkle remaining 1 tablespoon Parmesan over the crust.
• Bake until the crust is golden brown and potatoes are fork-tender, about 45 minutes. Transfer the sheet to a wire rack and let the tart cool for 10 minutes. Carefully slide the tart onto the wire rack and let it cool until just warm, about 20 minutes. Slice and serve warm.