Preheat oven to 375 degrees. Line a baking pan with parchment paper, and set aside. Have another baking pan, unlined, ready. In a small bowl, combine corn syrup with 1 teaspoon warm water; set aside.
Lightly dust a clean work surface with flour. Roll puff-pastry sheet into a 12-inch square; trim edges. Cut sheet on the fold lines, making three 4-by-12-inch rectangles. Place rectangles on lined pan. Top with another piece of parchment. Place the unlined pan on top of the filled pan. Bake puff pastry 8 minutes.
Remove the empty pan and top parchment, and return pan to oven. Bake until pastry starts to puff and is light-golden brown, about 8 minutes. Remove from oven; lightly brush with reserved corn-syrup mixture. Using a long metal spatula, gently flip rectangles. Brush again with corn-syrup mixture. Bake until shiny and golden brown, about 9 minutes more. With the spatula, transfer puff pastry to a wire rack to cool completely. Store in an airtight container until ready to use.
Increase oven temperature to 450 degrees. Using a pastry brush, coat the bottom of a medium oven-proof saute pan with 1 tablespoon butter; scrape vanilla seeds into butter. Place strawberries, cut sides down, in pan. Sprinkle with granulated sugar, and lay vanilla pod on top. Roast until strawberries start to "slump" and feel soft when squeezed, 10 to 12 minutes. Using a slotted spoon, transfer all but 2 of the berries and vanilla pod to a plate to cool. Reserve juices in pan.
Set pan over high heat. If using Grand Marnier, add to pan, and light a match, carefully igniting liqueur; allow flame to burn out, about 1 minute. Reduce heat to medium. Using a fork, mash 2 remaining strawberries; add orange juice. (If not using liqueur, add 2 tablespoons more orange juice). Cook until thick and syrupy, 30 to 40 seconds; discard vanilla pod. Remove from heat. Swirl in the remaining tablespoon butter, and set aside.
Combine cream and confectioners' sugar in the bowl of an electric mixer, and whip until stiff peaks form.
Without using a tip, close off the end of a pastry bag with a paper clip or clothespin; fill with whipped cream. Repeat, filling a second pastry bag with orange pastry cream. (Alternatively, use two resealable plastic bags, and cut off a corner on each; an offset spatula can also be used to spread both whipped and pastry creams.)
Pipe a tablespoon of pastry cream onto the middle of a serving plate. Center a rectangle of puff pastry on top. Leaving a 1/2-inch border, pipe orange pastry cream onto puff pastry. Arrange roasted strawberries on top of pastry cream; scrape any remaining juices from plate into reserved sauce. Place a second rectangle of puff pastry on top of strawberries. Leaving a 1/2-inch border, pipe whipped cream onto second rectangle of puff pastry. Sprinkle with pistachios. Place a third rectangle of puff pastry on top.
Using a serrated knife, slice assembled napoleon into 4 pieces. Drizzle with the reserved sauce, and serve.