The bakers of the really good millet bread keep their process a deep, dark secret, but I've heard it rumored that it's a sourdough process (so it's not truly yeast-free). I haven't got around to trying that yet. Rice flour is a main ingredient as well. (Also, it's processed on equipment shared with wheat bread, so although I can tolerate it well, it's not suitable for a challenge diet or for most people with celiac.)

Although the commercial bread lists flours, not whole grains, in the ingredient list, I think my next attempt will be to soak, grind and ferment whole millet and rice. That should eliminate some or all of the bitterness of the millet.