Prep the oven and pan: Preheat to 350°F (175°C). Line a 9x5-inch loaf pan with parchment, leaving overhang for easy lifting. Lightly grease the sides.

Whisk dry ingredients: In a large bowl, whisk almond flour, psyllium powder, coconut flour, baking powder, and salt until no clumps remain. This ensures even texture.

Mix the wet ingredients: In a separate bowl, whisk egg whites and whole eggs until frothy, about 30–45 seconds. Stir in melted butter (or oil), apple cider vinegar, and warm water.

Combine and rest: Pour wet into dry. Stir with a spatula until a thick, fluffy batter forms. It will look airy, not runny. Let the batter rest for 3–4 minutes so the psyllium hydrates.

Adjust consistency if needed: If the batter is too stiff to spread, add 1 tablespoon of warm water at a time. If it seems loose, let it rest an extra minute. It should be spreadable and hold soft peaks.

Fill the pan: Spoon batter into the lined loaf pan. Smooth the top with a damp spatula and score a shallow line down the center to help control the rise.

Bake: Place on the middle rack and bake for 40–50 minutes. The top should be golden and spring back to the touch. A toothpick should come out mostly clean, with just a few moist crumbs.

Cool properly: Cool in the pan for 10 minutes, then lift out using the parchment. Cool on a rack for at least 1 hour before slicing. This sets the crumb and prevents gumminess.

Slice and enjoy: Use a serrated knife to cut 12 slices. Toast if you like a crispy edge, and serve with butter, avocado, or deli fillings.