Preheat the oven to 350°F. Line a half sheet pan (baking sheet) with parchment.
Slice the craquelin bread into four 1” slices and place them on the prepared pan. Set aside to dry out for 30 minutes or so.
Make the simple syrup: Combine the sugar, water, and orange peel in a small saucepan. Set the pan over medium-high heat and bring the syrup to a simmer, stirring to help dissolve the sugar. Adjust the heat to keep the syrup simmering for 3 minutes.
Remove the syrup from the heat and pour it through a fine-mesh strainer into a heat-safe bowl to remove the orange peels. Set aside.
Make the pistachio frangipane: Put the toasted pistachios, sugar, and salt in the work bowl of a food processor and process until the pistachios are coarsely ground.
Add the remaining ingredients to the food processor and continue to process, scraping the bowl as necessary, until a smooth, creamy paste forms; it should be about the texture of your favorite chunky nut butter. Set aside.
Assemble the bostock: Generously brush the orange syrup onto both sides of each slice of bread; you should use up all the syrup.
Spoon or pour the pistachio frangipane onto the top of each slice of bread, dividing it evenly among the four slices. Spread the frangipane so that it covers each slice evenly.
Sprinkle the chopped pistachios and orange zest onto each slice, again, dividing the nuts and zest evenly among the slices.
Bake the bostock for 15 to 20 minutes until the frangipane has set and looks golden around the edges.
Remove the bostock from the oven and allow to cool for 15 to 20 minutes before serving. Dust lightly with confectioners’ sugar, if desired.