Preheat oven to 350 F and prepare a baking sheet with parchment paper. In a bowl, stir together the flax meal and water and then set aside for 5 minutes.
In a microwave safe bowl, melt the butter and then stir in the agave and vanilla. Then add the mixture to the bottom of a stand alone mixer. Add in the almond flour, gluten free flour and salt. Finally add in the flax meal and water mix and using a paddle attachment, stir until a dough forms. Ultimately, you can do this by hand in a large mixing bowl or in a food processor.
Separate the dough into two large balls. Place one between two pieces of parchment paper and roll them out until they are about 1/8 inch in thickness. Use a linzer cookie cutter to cut out cookies and shapes. Transfer the cutouts to the parchment paper and bake for 7-9 minutes. Repeat until you have used all the dough.
for the filling
Add your tapioca flour and water to a small pot and whisk until smooth. Turn on low heat and add in the lemon juice, zest, agave and salt and continue whisking until the mixture thickens into a curd, about 1-2 minutes. Repeat this step with the grapefruit.
Place a small spoonfuls of the curd between two cookies and then dust with powdered sugar. Serve!