Preheat oven to 350 degrees and lightly grease an 8x4-inch or 7x4-inch loaf pan with avocado oil or olive oil. Line the bottom and sides of the pan with parchment paper so the ends hang over the sides. Don't skip the parchment paper or the bread will stick!
In a large mixing bowl, whisk together the blanched almond flour, ground flax seeds, sea salt, baking powder, xanthan gum, and sweetener (if using).
Melt the butter, then beat or stir into the dry ingredients. Set aside.
In a large mixing bowl, combine the egg whites and cream of tartar (if using). Be sure the bowl is large enough because the egg whites will expand a lot. Using an electric mixer, beat the egg whites on high until stiff peaks form.
Note- If you used the mixer to beat the butter into the dry ingredients, clean off the beaters before using them.
Scrape half of the egg whites into the batter and beat until just combined. Be careful not to over-mix!
Then, add the batter to the bowl with the remaining egg whites and gently fold until there are no more streaks, resisting the urge to stir! Try to keep the egg whites as intact as possible.
Transfer the batter into the lined loaf pan and smooth out the top. Gently press the batter towards the middle of the pan to create a rounded top.
Bake 45 to 50 minutes, or until the top is golden brown. Then, tent the top with aluminum foil and bake another 15 to 25 minutes, or until the top is firm and does not make a squishy sound when pressed in the middle. The internal temperature should be 200 degrees.
Allow to cool in pan 10 to 20 minutes, then transfer to a cooling rack to cool completely, removing the parchment paper. Be sure to cool completely before slicing.