Heat the oven to 375° F.
Line a baking sheet with parchment paper.
In a large bowl, combine the almond flour (sifting through a fine-mesh sieve if it's lumpy), arrowroot, baking powder and salt. If using fennel seeds or orange zest, add them now. Whisk everything together until combined.
In a medium bowl, whisk the egg, egg white, maple syrup and vanilla until combined.
Pour the wet ingredients into the dry ingredients. Use a spatula to stir everything together. Then, if using sliced almonds or chocolate chunks, stir them in. The dough will be somewhat thick and sticky.
Transfer the dough to the lined baking sheet. Using the spatula or your hands, push and spread the dough into a rectangle roughly 9x3". If the dough is sticky, get your hands slightly damp and then form it into shape. Use a paper towel or towel to dab up any drops of water on the baking sheet.
Bake the dough for 25 minutes, until it's puffed up, has spread a bit and become golden brown. Remove from the oven (leave the oven on) and let it rest on the baking sheet until cool enough to handle, about 10 minutes.
Use a large spatula to carefully transfer the baked dough to a cutting board. Use a long serrated/bread knife to cut roughly ½-inch slices on an angle. Gently arrange the slices on the baking sheet with one cut side facing up.
Bake again for 10 minutes. Remove from the oven and use the spatula or serrated knife to gently flip the cookies onto the other cut side. Bake one last time for 5-10 minutes, until a darker golden brown. Let cool completely on the baking sheet.