Mix the dry ingredients: In the bowl of a stand mixer, combine 200 g brown rice flour, 145 g tapioca starch, 95 g potato starch, 66 g granulated sugar, 10 g kosher salt, 9 g instant yeast, 7 g xanthan gum, and 10 g baking powder. Whisk to combine.
Make the psyllium gel: In a small bowl, whisk together 20 g whole psyllium husk and 200 g warm water until thickened into a gel. Set aside for about one minute to hydrate fully.
Combine the wet ingredients: In a medium bowl, whisk together 215 g warm whole milk, 1 large egg, 70 g avocado oil, and 6 g apple cider vinegar. Add in the psyllium gel and whisk until smooth and uniform.
Make the dough: Pour the wet ingredients into the dry ingredients. Using the dough hook attachment, mix on medium speed for about 5 minutes, scraping the sides and bottom of the bowl as needed. The dough should be soft, thick, and slightly sticky.
Knead the dough: Dust your hands and work surface with tapioca starch. Turn out the dough and knead by hand for 1 to 2 minutes until smooth.
Shape the dough: Pat the dough into a rectangle the width of the loaf pan, about 1 inch thick.
Roll it into a log: Roll the dough up tightly from one short end into a log, pinching the seam closed.
Set it in the pan: Place the log seam-side down in a greased 9x5-inch metal loaf pan.
Proof the dough: Cover the pan loosely and let rise in a warm spot (75–80°F) for about 60–90 minutes, or until doubled in size and the top is just above the rim of the pan. In a cold kitchen, let it rise in the oven with the oven off and the light turned on.
Bake: Preheat the oven to 375°F (190°C) during the last 30 minutes of the rise time. Brush the top with 1 large egg that has been beaten. Bake for 55–65 minutes, or until deep golden brown and the center reaches 210°F. Tent with foil if the bread is browning too quickly.
Cool: Cool in the pan for 15 minutes before transferring to a wire rack to finish cooling for a full 1 hour before slicing.