Heat oven to 350° F. Butter a 9-inch fluted removable-bottom tart pan or round cake pan.
In a food processor fitted with the metal blade, process the flour, sugar, butter, and salt until moist clumps form. Add the orange zest and poppy seeds and pulse once or twice just to combine.
Press the mixture evenly into the prepared pan. Bake until lightly golden, 25 to 30 minutes.
Using a serrated knife, cut the warm shortbread into 12 wedges. Let the wedges cool completely before removing them from the pan.