Adjust oven rack to middle position and preheat oven to 425°F (220°C). Set a 13-by-18-inch rimmed baking sheet upside down on middle rack to preheat. Line another 13-by-18-inch baking sheet with parchment paper.
On a lightly floured surface, roll puff pastry into a 9-by-12-inch rectangle. Pick up the pastry by carefully rolling it around your rolling pin, using a bench scraper to lift it cleanly from the work surface, if needed. Unroll it onto the parchment-lined baking sheet.
Using an offset spatula, spread the cream cheese evenly across puff pastry, leaving a 1-inch border on all sides. Using the tip of the spatula, create small swoops and divots. Add pepper jelly in a similar fashion, swirling it into the cream cheese to create a textured surface. Using a paring knife, score a 1-inch border around perimeter of the cream cheese and pepper jelly filling, and brush with egg wash. Lightly season border with salt and pepper. Freeze the tart until firm, about 15 minutes.
Place baking sheet with tart on top of preheated baking sheet and bake until edges are deep golden brown and puffed, and the cream cheese and pepper jelly have melted together and thickened slightly, 14 to 16 minutes.
Garnish with scallions and sesame seeds, if desired. Cool slightly, about 5 minutes. Using a serrated knife, cut into rectangles. Serve immediately.
