Make the yeast-psyllium mixture: Sprinkle the yeast into a large bowl. Add the water, milk, and honey and whisk until dissolved. Let the mixture sit until frothy, about 10 minutes. Whisk in the psyllium. Let it gel for 5 minutes.

Make the dough: In a stand mixer, stir together the sorghum flour, tapioca starch, potato starch, salt, and baking powder. Add the yeast-psyllium mixture and butter to the dry ingredients. Snap on the dough hook and mix on medium-low speed until the dough comes together, 2 to 3 minutes. It will be moist but hold together nicely.

Shape and proof the dough: Transfer the dough to a work surface and knead a few times to shape into a tight ball. Roll this ball into an oval shape that is about 8 inches (20 cm) long. Grease the bottom and sides of a 9-inch (23 cm) Pullman loaf pan as well as the bottom and underside part of the lid with butter. Place the dough inside and cover the pan with a kitchen towel. Proof the dough until it almost reaches the top rim of the pan, about 30 minutes. Don't let it go past the top of the pan or it can overproof and collapse while baking.

Preheat the oven: Position a rack in the bottom third of the oven and preheat the oven to 425°F (220°C).

Bake the bread: Cover the Pullman pan with its lid. Bake for 30 minutes. Reduce the oven temperature to 375°F (190°C) and bake for 30 minutes. Turn the oven off, remove the lid, but leave the pan in the oven for 30 minutes.

Cool the bread: Invert the bread onto a wire rack and let it cool for at least 30 minutes before cutting. It is best eaten the same day, but it is also great toasted a couple of days after. Store at room temperature wrapped in a paper bag for up to 3 days.