Preheat the oven to 350F. Line a large baking sheet with parchment paper.
Whisk together the ground flaxseed and water in a medium bowl. Let stand for 10 minutes to thicken into a flax egg.
Meanwhile, add the raw almonds to a food processor and processor for 30 to 45 seconds, or until you have a coarse flour/fine meal and all of the nuts are thoroughly ground, stopping to pulse several times to ensure even processing. Be careful not to over-process or you'll end up with almond butter.
Transfer the almond flour to a large mixing bowl, and add the arrowroot and sea salt. Whisk together until combined. Stir in the dried cherries, cacao nibs (if using), and sliced almonds.
To the flax egg, add the pure maple syrup and vanilla extract. Whisk until combined. Pour over the almond flour mixture and use a large wooden spoon to mix until combined. At first it will seem like there isn't enough liquid—have faith and keep stirring until the liquid disperses evenly throughout the entire mixture.
Turn the "dough" out onto the lined baking sheet, and divide it into two equal pieces.
Use your hands to shape and compact the dough into 2 tightly packed rectangles. Each rectangle should be approximately 4 inches wide x 6 inches long, and just shy of 1 inch thick. If the dough is sticking to your hands, lightly dampen them with water to repel the mixture.
Bake for 20 to 24 minutes, or until the edges are light golden-brown and each rectangle feels well-set, yet retains a soft indentation when gently pressed. Remove from oven and cool for 20 minutes.
Meanwhile, decrease the oven temperature to 275F.
Once the biscotti rectangles are mostly cool, use a sharp knife to cut them widthwise into ¾-inch-thick slices, pressing straight down and rocking the knife back and forth to slice (rather than using a sawing motion). You should have 16 biscotti, 8 from each rectangle, and each should be approximately 4 inches long.
Carefully return the biscotti to the lined baking pan, sliced-side down.
Bake for 14 to 18 minutes. Then, use a metal spatula to carefully flip each biscotti, and bake for another 14 to 18 minutes, or until rich golden brown and crisp to the touch. They'll continue to crisp as they cool, so keep that in mind when testing for doneness.
Carefully transfer the biscotti to a wire cooking rack. Cool completely.
If dunking in chocolate (optional): Prepare the chocolate glaze while the biscotti cool. Melt the chocolate chips in a double boiler or Bain Marie (or in the microwave if you prefer) until smooth and glossy, being careful not to overheat.
Once the biscotti are cool, dunk one side of the biscotti into the chocolate and return to the parchment-lined pan. Garnish with shaved almonds (or pepitas for nut-free) if desired and let cool.
Store the biscotti in an airtight glass container to maintain crunchiness. (Note: plastic containers or bags can cause the biscotti to lose their crispy snap and bite.)