Step 1 In a small bowl, combine the currants, citron, limoncello and lemon zest in a small bowl. Set aside to plump for 20 minutes while you prep the other ingredients, stirring once or twice in between.
Step 2 In a large bowl, whisk together the spelt flour, sugar, baking powder and salt in a large bowl. Make a well in the center. In a medium bowl, lightly whisk the eggs and the vanilla extract until blended, then gradually add the honey, whisking until the mixture is smooth, 30 to 60 seconds. Add the egg-honey mixture to the center of the dry ingredients.
Step 3Using a hand mixer on low speed, beat the ingredients until just combined (the dough will look crumbly). Do not overmix. Add the plumped dried fruit with their liquid and the pistachios, and beat until blended. Switch to a wooden spoon if the dough becomes too stiff, then give the dough one final stir. Cover the bowl with a plate and let sit at room temperature for 30 minutes.
Step 4Meanwhile, position a rack in the center of the oven and preheat to 325 degrees. Line a large cookie sheet with parchment paper. Have a small bowl with water ready.
Step 5Divide the dough in half. It will be heavy and sticky. Place the cookie sheet in front of you so that the short side faces you. Scrape one half of the dough onto the top third of the cookie sheet, creating a rough oblong mound. Moisten your hands with water and shape the mound into a flat log, about 7 inches long and 4 inches wide. Repeat with the second half of the dough, placing it on the bottom third of the cookie sheet, allowing at least 2 inches between logs as they will spread a fair bit. Brush the tops and the sides of the logs with egg white and generously sprinkle with turbinado sugar.
Step 6Bake until the tops turn golden brown, the edges darken and the logs firm up, 25 to 30 minutes. Carefully slide the logs with the parchment paper onto a wire rack and set aside until they are cool enough to handle, about 15 minutes. Leave the oven on and decrease the temperature to 300 degrees.
Step 7Transfer the logs to a cutting board. Gently pull off the parchment paper and discard. Using a long, sharp, serrated knife, cut each log diagonally into 1/2-inch thick slices. Return the slices to the same cookie sheet (no need for parchment), placing them upright and leaving about 1/2 inch in between.
Step 8Bake until the biscotti feel dry to the touch at the cut sides (not on top) and just start to brown at the edges, 17 to 20 more minutes. They will crisp as they cool.