In a large mixing bowl, whisk together dry ingredients. Set aside.
Grind the flax seeds in a coffee grinder. It will yield approximately 4 tablespoons of ground flax seeds (a.k.a. flax meal). Transfer the ground flax seeds into a small mixing bowl. Add water and let it swell to make flax egg. Approximately 5 minutes.
Add the remaining wet ingredients into the flax egg and whisk until well combined.
Add the wet ingredients to the dry ingredients and gently mix with a wooden spoon or clean hand until well combined to form a dough.
Transfer the dough onto the prepared baking sheet. With wet hands, pat the dough into a rectangle about ½” thick. Spray the dough with cooking spray.
Bake the dough for 20 minutes until the top of the dough is slightly browned. Remove it from the oven. Heat the oven to 400 °F. Place the baking sheet on a cooling rack for approximately 5 minutes to let the dough cool slightly. Transfer the dough onto a cutting board and use a sharp knife to cut it into 1” wide slices. Set the slices, cut side down, on the baking sheet. Return the slices to the oven to bake for 18 minutes and turn them over half way.
Remove the baking sheet from the oven and let the biscotti cool completely on a wire cooling rack before serving.
1. You may use store-bought all-purpose gluten-free flour. 2. These biscotti can be stored in airtight container up to 10 days.