How to Make It
Heat oven to 400°F and position a rack in the upper third of oven. Place walnuts on a rimmed baking sheet and toast, stirring halfway through, until fragrant and slightly darkened in color, 6 to 7 minutes. Let cool. Increase oven temperature to 450°F.
In a food processor, pulse walnuts until finely ground. Transfer to a medium bowl and add the zest. Heat butter, honey, and ⅛ tsp. salt in a medium skillet over medium, stirring constantly, until butter is melted, about 1 minute. Stir into walnut mixture until well combined. Let cool.
In a small bowl, mix together sugar and ⅛ tsp. salt. Scatter mixture on a clean work surface to form a 14 x 12-inch rectangle. Place puff pastry directly over sugar, then roll to a 14 by 12-inch rectangle. Using your fingers, gently and evenly spread cooled nut mixture over puff pastry.
Fold in the two outer long sides of the dough to meet in the center. Repeat the same fold by bringing the long sides to the center again. Fold one half of the dough over the other lengthwise and transfer to parchment-lined baking sheet (you may have to lay it diagonally to fit). Place in freezer about 15 minutes (this will firm the dough and make it easier to slice).
Slice crosswise into ½-inch-thick slices and bake, flipping every 5 minutes, until golden brown, about 12 minutes. Transfer to a wire rack to cool completely. Store in an airtight container up to 2 days.