Preheat oven to 350° with rack in center. Spray 4 x 8-inch slant-sided bread pan (bottom measures 3.5 x 7.5-inches) with nonstick cooking spray. Line pan with two strips of parchment paper, crisscross, so about 1 inch of paper overhangs on all sides of pan. This will keep sides of bread soft and allow for easier removal of baked loaf from pan. Spray one more time with nonstick cooking spray.
In large mixing bowl, whisk eggs until well blended. Add honey, oil, and vinegar to the bowl all at once, and mix well again. In a separate medium bowl, whisk together Almond Flour, Coconut Flour, chia seed, flax seed, baking soda, cream of tartar, and salt. Add flour mixture to egg mixture, and whisk together until there are no lumps. Batter will be runny.
Pour batter into loaf pan and bake 38 to 48 minutes, until a long toothpick or thin skewer inserted near center comes out clean. After it comes out clean, bake 2 to 3 more minutes to prevent a soft spot in the center near the top. Bread will be pulling away from the sides of the pan. Cool bread in pan on wire rack 20 to 30 minutes. Remove by lifting out with parchment paper.
Serve warm, room temperature, or toasted.
This bread stays moist for about 3 days; it keeps best wrapped in parchment paper, inside an open plastic bag.
NOTE: Small, slant-sided pan is specified to make the tallest loaf possible. A wider pan will work, but will make a shorter loaf.