1. Preheat the oven to 350°F (175°C). Line a 9×5 inch loaf pan with parchment paper, leaving some overhang on the long sides, and lightly grease any exposed areas.

2. Separate the eggs, placing the whites into a clean mixing bowl and the yolks into a second large bowl.

3. Using a hand mixer or stand mixer, beat the egg whites with a pinch of the salt until stiff, glossy peaks form. Set aside.

4. To the bowl with the egg yolks, add the Greek yogurt, melted butter or avocado oil, apple cider vinegar, and the optional granulated sweetener. Whisk until the mixture is completely smooth.

5. Sprinkle the whey protein isolate, psyllium husk powder, baking powder, and remaining salt over the yogurt mixture. Stir or whisk until the dry ingredients are fully incorporated and a thick batter forms.

6. Gently fold one third of the beaten egg whites into the batter to lighten it. Once mostly combined, fold in the remaining egg whites in two more additions, using broad, gentle strokes to preserve as much air as possible.

7. Transfer the batter to the prepared loaf pan and smooth the top with a spatula. If using the egg wash, lightly brush the surface of the batter with the yolk and water mixture.

8. Bake on the center oven rack for 32 to 38 minutes, or until the loaf is deeply golden and a toothpick inserted into the center comes out clean or with just a few moist crumbs. Tent loosely with foil if the top browns too quickly.

9. Remove the pan from the oven and let the bread cool for about 10 minutes. Lift the loaf out using the parchment overhang and transfer it to a wire rack.

10. Allow the bread to cool almost completely before slicing into 12 pieces. Serve plain, toasted, or however you love your low carb bread.