Line a standard-size metal loaf pan with parchment paper and spray it with cooking spray; set it aside.
Add the sugar and yeast to the warm water (95–110°F) and stir. Set aside for 5–10 minutes but no longer, until foamy.
Mix the dry and build the batter
While the yeast is proofing, add the flours, flax seed meal, xanthan gum, baking powder, and salt to the bowl of a stand mixer fitted with the paddle attachment. Mix on low just until combined.
With the mixer still going, add the oil, egg whites, vinegar, and the proofed yeast mixture.
Increase to medium speed and mix for 2 minutes. The dough will be thick and sticky, closer to a thick cake batter than a kneadable dough.
Shape and rise
Use a rubber spatula to add the dough to the prepared loaf pan, filling the corners and leveling the top. Wet your fingers and smooth the top.
Cover with a lightly oiled piece of plastic wrap and let rise in a warm place for 45 to 60 minutes, or until it has risen slightly above the loaf pan.
When the dough is near the top of the pan, preheat the oven to 350°F.
Bake and cool for the best crumb
Remove the plastic wrap and bake for 60–65 minutes. Halfway through baking, cover the loaf with foil to prevent over-browning.
Remove from the oven and let cool completely before slicing.
Slice the entire loaf and store in a container at room temperature for up to 4 days, or freeze for up to a month.
Notes
For the best texture, bake in a metal pan, keep the yeast proof to 5–10 minutes, and stop rising once the dough is only slightly above the rim. Cool completely before slicing to avoid a gummy crumb.Nutrition information is estimated based on common ingredients and serving sizes and may vary.