Preheat the oven to 350°F.
Line a heavy large baking sheet with parchment paper.
Whisk flour, salt and baking powder in a medium bowl to blend. Using stand mixer (or a hand-held mixer), beat the sugar and butter until fluffy, about 2 minutes. Add the eggs and continue to mix, then add the lemon and orange zest until blended.
Add about 1/3 of the flour, on low speed.
Add the nuts and cranberries and then add more of the dry ingredients, a little at a time.
Form the dough into a long, wide log on the prepared baking sheet.
Bake until light golden, about 40 minutes.
Place the log on the cutting board. Using a sharp, serrated knife, cut the log on a diagonal into 3/4-inch thick slices.
Note: I found that cutting straight down, rather than sawing, made for less crumbs or broken slices.
Arrange the biscotti, cut side down, on the baking sheet.
Bake the biscotti until they are pale golden, about 15 minutes. Transfer the biscotti to a rack and cool completely.
Note: I baked mine at 325°F and turned them over after 7 minutes. My biscotti turned the way I prefer them to be—lightly golden, lightly crispy on the outside, and soft on the inside. If you prefer yours to be hard and crunchy, bake at 350°F.