1. Wash and slice the rhubarb and strawberries.
2. Mix the water and flax seed together, whisking well to prevent lumps forming. Let this mixture sit while preparing the rest of the ingredients.
3. Blitz the hazelnuts in a blender to create flour.
4. In a medium bowl beat the butter and sugar for 1 to 2 minutes until creamy and paler in colour. Beat in the egg, flaxseed mixture and then the hazelnut flour, quinoa flour, white rice flour, oat flour, baking powder and salt. until there is mo loose flour.
5. Cover and chill the dough for 20 to 30 minutes.
6. Sprinkle tapioca starch over parchment paper and roll out half the dough with a rolling pin. Add as much tapioca starch as necessary to prevent sticking.
7. Place the dough for the base still on the parchment paper and press it into the edges of a pie plate with your fingers. You may need to trim the paper a little.
8. Roll out the other half of the dough for the top and chill for 10 minutes or so.
9. Bake the empty pie crust at 400°F for 6 minutes.
10. Fill the pie and bake at 400°F for 20 minutes. Reduce the heat to 375°F and bake for 10 -12 minutes, keeping a close watch to see that the pastry does not over brown.
11. Let the pie cool for about 20 minutes before eating.