Preheat the oven to 375 degrees F. Butter a baking sheet and dust it with sweet rice flour.
Whisk the sweet rice flour, sorghum flour, almond flour, teff flour and potato flour together to aerate and incorporate them. Mix in the oats, flaxseed, baking soda and salt. Toss in the currants.
Make a well in the center of the dry ingredients and pour in 2 cups of the buttermilk. Mix the buttermilk into the flour mixture, gradually adding more buttermilk as necessary, until the dough is moist and soft but not wet, with no visible flour left. You will probably use about 3 cups, but feel free to use less or more, depending on your kitchen.
Turn the dough out onto the prepared baking sheet. With lightly floured hands, shape the dough into a round about 3 inches thick. Cut a deep cross into the top of the loaf with a wet, serrated knife. Beat the egg with the water in a cup. Brush the egg wash evenly onto the loaf.
Bake until the crust is dark brown and you hear a hollow thump on the bottom when you tap it, 45 to 60 minutes. Transfer the bread to a rack. Serve warm or cool completely.