Whisk together the flour, baking powder and salt in a medium bowl.
In a stand mixer, beat the sugar, butter, and lemon zest until fluffy. Beat in the eggs one at a time. Add the flour mixture in two additions to avoid a flour flurry, and beat just until blended. Add the pistachios, cranberries and white chocolate, mix until just blended.
Transfer the dough to the prepared baking sheet and form into a 13-inch long and 3-inch wide log. Bake until light golden, about 40 minutes. Cool for 30 minutes.
Place log on the cutting board. Using a sharp serrated knife, cut the log on a diagonal into 1/2 to 3/4-inch-thick slices. Arrange the biscotti, cut side down, on the baking sheet.
Bake the biscotti until they are pale golden, 15 to 20 minutes. Then transfer to a wire rack to cool completely.
Cooled biscotti will keep in an airtight container at room temperature for up to 2 weeks.