Mix the flour, rolled oats, baking powder, baking soda, and salt in a small bowl. Whisk the melted butter and sugars in the bottom of a large bowl. Whisk in the eggs and vanilla. Stir in the dry ingredients, nuts, coconut, and dried fruit. Expect a stiff batter.
Position a rack in the center of the oven, and heat to 325 degrees. Line a baking sheet with parchment paper.
On a floured counter, using floured hands roll half the dough into a log a little shy of the length of your baking sheet, 12 to 14 inches. Transfer the dough log to the baking sheet, and pat lightly until it becomes more oval-shaped. Repeat with the second half of the dough. Beat the egg white until foamy, and brush it over logs. Bake the logs for 20 minutes, until they are lightly golden brown and beginning to form cracks.
Let cool almost completely (it’s okay if the centers are still lukewarm), about 1 hour. With a serrated knife, cut the logs on the bias into ½ -inch-thick slices. They will be crumbly; cut as gently as possible. Transfer the slices back to the parchment-lined baking sheet, and lay flat in a single layer. Bake for another 20 minutes, until toasted and crisp. (If you like, you can flip them halfway for more even browning, but you will have good color on them either way.) Cool the biscotti on the baking sheet, or transfer to a rack.
Note: This recipe should prove very tweakable; you could use cinnamon, or almond extract, add citrus zest, vary the fruits and sweeteners. You could swap half the flour for whole wheat or even oat flour. Or you could add some chocolate chips. Who could blame you?
Do ahead: Biscotti keep in an airtight container at room temperature for up to 2 weeks, and longer if well wrapped in the freezer.
Excerpted from Smitten Kitchen Every Day: Triumphant and Unfussy New Favorites. Copyright © 2017 by Deb Perelman. All rights reserved. No part of this excerpt may be reproduced or reprinted without permission from the publisher.
Find more recipes from Deb Perelman here.