Grease a loaf pan that is no more than 9 x 5 inches and set it aside.
In the bowl of your stand mixer fitted with the paddle attachment, place the flour, xanthan gum, teff, oat flour, cream of tartar, baking soda, sugar and yeast. Whisk with a separate handheld whisk to combine well. Add the salt and whisk again to combine.
Add the butter, shortening, vinegar, molasses, egg whites and milk, and mix on low speed with the paddle attachment until the dough starts to come together. Cover the mixer with a tea towel and mix high for about 5 minutes. The dough should be thick, but not quite as stiff as french bread dough.
Transfer the dough to the prepared loaf pan, cover with plastic wrap and place in a warm, draft-free area to rise until the dough is about 150% of its beginning volume. While the dough is rising, preheat your oven to 375 degrees F.
Once the dough has risen, remove the plastic wrap and place the bread in the center of the preheated oven. Bake for about 30 minutes or until the loaf is firm enough to take out of the pan. Remove the bread from the loaf pan and place it on a rimmed baking sheet. Return the bread on the pan to the oven, and bake for another 5 to 10 minutes, or until the top is nicely browned and the loaf sounds hollow when tapped. Remove from the oven, allow to cool on the baking sheet for 10 minutes. Transfer to a wire rack to cool completely before slicing and serving.

Notes:
Please make the recipe as directed, following the ingredients and instructions to the letter, taking care not to substitute your judgment for mine until you are very comfortable with the recipe or it will likely not turn out as we both hope.

I have not tested this recipe with any substitutes at all.
King Arthur gluten-free flour will not work in this recipe as it is relatively useless for yeasted breads.

I make my own oat flour by processing gluten-free old-fashioned rolled oats in the food processor until they’re finely ground.