Preparation

• Line a large rimmed baking sheet with parchment.
• Heat the oil in a large nonstick skillet over medium-low heat. In batches, cook the pancetta, flipping once until just cooked through, about 3 minutes. Use tongs to transfer the pancetta to a plate lined with paper towels. Do not clean the skillet. Add the onion, rosemary, 1/4 tsp. salt, and 1/4 tsp. pepper to the skillet, and cook, stirring occasionally, until the onion is very soft and just beginning to brown on the edges, about 10 minutes. Transfer to another plate lined with paper towels.
• Lightly flour a clean work surface. Unfold the puff pastry sheets, lightly flour them, and gently roll each into a 10-inch square. Transfer one of the squares to the prepared baking sheet, and spread the mustard over it, leaving a 1-inch border on all sides. Arrange the potato slices over the mustard in a slightly overlapping single layer. Arrange the pancetta on top, breaking slices as necessary to fit. Sprinkle the onion-rosemary mixture evenly on top of the pancetta. Sprinkle the Gruyère evenly on top of the onion mixture.
• Brush the borders of thepastry with the beaten egg, then cover with the other pastry sheet; it doesn’t have to be perfectly aligned. Press gently around the edges to seal. Brush the rest of the beaten egg over the entire pastry. Use a sharp knife to cut four 2-inch diagonal slits on top for steam to escape, then sprinkle with the sesame seeds. Refrigerate for at least 20 minutes and up to 12 hours.
• Meanwhile, position a rack in the center of the oven, and heat the oven to 425°F. Trim the edges of the pastry with a sharp knife or pizza wheel. Bake, rotating the pan halfway through, until puffed and golden brown, 25 to 30 minutes. Let cool 10 minutes before serving.