Preheat oven to 350°F and line a baking sheet with parchment paper.
Prepare the flax egg by mixing ground flaxseed meal with water. Let it thicken for about 5 minutes.
In a bowl, combine dry ingredients.
Add wet ingredients, including the flax egg, and mix well.
Form the dough into a ball and transfer it to the baking sheet.
Use a greased spatula to press the dough into a 1/2-inch thick cookie.
Bake for 13-15 minutes until golden brown.
While baking, prepare the icing by mixing lemon juice and powdered sugar or monk fruit sweetener until smooth.
Once the cookie is done, let it cool completely.
Top with icing and garnish with poppy seeds and lemon zest, if desired.
Serve and enjoy your delicious giant cookie!