Make the tart crust. Place tapioca flour, cornstarch, sweet rice flour, millet flour, salt, guar gum and sugar in the bowl of a stand mixer fitted with the paddle attachment. Beat lightly to combine. Add the butter, and mix on medium speed until the mixture becomes pebbly. Add the eggs and beat just until the mixture turns on itself. Turn out onto a board that's been lightly floured with tapioca flour and knead for one or two turns. Flatten dough into a disk, wrap in plastic and refrigerate for at least 2 hours (it can be stored for a few days in the fridge).

Remove dough from fridge and place on a board that's been lightly floured with tapioca flour. Roll out into a 12-inch circle. Using your rolling pin, carefully roll up the dough and transfer to a 9-inch tart pan with a removable bottom. Press dough into the pan, taking care not to stretch too much. Remove excess dough from the edges. Prick dough all over with a fork. Spray the shiny side of a large piece of foil with GF cooking spray and lightly press onto dough. Fill with pie weights (I used a very old one-pound pinto beans). Place pan in the freezer for 30 minutes while you preheat the oven to 375 degrees.

Remove from freezer and place directly into the oven. Bake for 20 minutes. Remove the foil and pie weights and bake for an additional 20 minutes or until the crust is lightly golden brown. Remove from the oven and cool completely. (Can be covered tightly and kept at room temperature overnight)

Make the lemon curd. Remove the zest from the lemons using a vegetable peeler, taking care to use only the yellow part of the peel, not the white pith. Place into the bowl of a food processor fitted with a steel blade attachment. Add the sugar and pulse until the zest is chopped into tiny bits and thoroughly combined with the sugar. Add the butter and pulse until smooth. Add the eggs, one at a time and pulse until smooth. Add the lemon juice and salt and pulse again.

Pour the mixture into a medium saucepan set over low heat. Cook for 10-15 minutes, stirring constantly, until the mixture is quite thick...like a soft-set pudding.

Pour the mixture through a sieve into a bowl or pitcher, then pour the warm curd into the cooled tart crust. The curd will set at room temperature, or you can refrigerate for a firmer texture. Top with fresh raspberries, if desired.