Grease a square baking pan (I use coconut oil) and set it aside for now.
Place the blueberries in a bowl. Sprinkle in the lemon juice, lemon zest, and vanilla extract. Stir them together.
In a small dish, mix the coconut sugar, arrowroot flour, and salt. Sprinkle this over the blueberries and stir to coat.
Spread the mixture into the prepared baking dish.
In another bowl, whisk together the almond flour and arrowroot flour. Add in the chopped walnuts, maple syrup, and melted coconut oil. Stir this together to create a crumbly mixture.
Sprinkle the mixture over the top of the blueberries.
Bake the dessert for 40-45 minutes, or until the blueberries start to bubble and the crust begins to get golden.
Serve as is or with whipped cream/ ice cream if you wish.