Put the sugar, honey, orange blossom water, egg white, vanilla, baking powder, and salt in a medium bowl. Stir with a rubber spatula to thoroughly combine. Add the pistachios and stir to coat the nuts in the syrup.
Remove the tart pan from the oven and quickly distribute the nut topping on top. Return the tart back to the oven and finish baking for about 15-20 minutes until the nut topping is golden brown, rotating the tart halfway to ensure even browning. Remove the tart from the oven and set it aside to cool to room temperature.
To serve, pour the remaining powdered sugar for dusting into a fine-mesh sieve and tap the sieve over the tart to dust it lightly. Gently push the removable bottom to lift the tart out. Using a long offset spatula, gently slide the tart from the metal bottom onto a long platter or wooden board.