Preheat oven to 350 degrees F (175 degrees C). Grease a baking sheet.
Whisk water and ground flax seeds together in a small bowl. Let stand until thickened, about 5 minutes. Mix in maple syrup, coconut oil, lemon zest, lemon juice, and vanilla extract.
Mix oat flour, white rice flour, brown rice flour, coconut flour, poppy seeds, baking soda, baking powder, xanthan gum, and salt together in a large bowl. Add maple syrup mixture; mix with a spatula until dough comes together in a ball.
Transfer ball of dough to a large sheet of waxed paper; press into a circle about 1 inch thick. Cut into 8 wedges. Pull wedges apart and carefully transfer to the prepared baking sheet.
Bake in the preheated oven until golden brown, 15 to 17 minutes. Cool on a wire rack.