I learned how to make this from America's Test Kitchen, but have changed it to make a smaller, slightly less rich loaf. The recipe works well in a deep (high sided) loaf pan or a Pullman. Makes a 1 pound 10 oz loaf (26 oz).
1tablespoonunsalted butter, very soft but not melted(14 grams)
Instructions
Heat the oven to 200 degrees and then turn it off. The goal is to create a very warm proofing box. You don't have to do this, but it sometimes speeds things up.
In the bowl of a stand mixer, combine the gluten-free flax blend, psyllium powder, instant yeast, sugar, baking powder and salt. Stir until blended.
Heat the water in the microwave for about a minute and a half or just boiling, and let it cool down to 125 degrees F.
Add the 12 oz of 125 degree water to the flour mixture and stir just until moistened, then add the egg, egg white and the butter. Using the paddle attachment, beat on medium speed for about 3 minutes. The dough should clear the sides of the bowl and form a mass on the paddle. If it sticks to the side of the bowl, just scrape it off. It should still be slightly moist, but dry enough so that you can scrape it onto a mat and bounce it a little.
Scrape onto a pastry mat (you may or may not need a tiny bit of flour, I never do) and press down. Fold over on itself once or twice and shape into a compact loaf. Transfer to a greased Pullman pan.
Cover with greased plastic wrap, then put in the warm oven (make sure you turned it off) for 20 minutes. Remove from oven and let rise for another 20 minutes or until loaf has risen to the top of the Pullman. Meanwhile, preheat oven to 350 degrees F.
Bake risen loaf at 350 degrees F. for about 70 minutes. Turn off the oven. Let cool slightly and carefully remove the loaf from the pan. At this point, if it feels kind of heavy (which is usually will), let it sit in the oven with the door closed to further dry out the crumb for about 20 minutes. Remove from the oven and let cool completely.
The loaf might seem heavy at first, but as it stands and cools it will tend to solidify more. Once cool, you should be able to slice it. It should be firm enough to hold light sandwich fillings only, but it's mostly best for toast.