Preheat oven to 350°F (175°C). Line a 9x5-inch loaf pan with parchment. Whisk almond flour, coconut flour, psyllium, baking soda, and salt in a large bowl.

In a separate bowl, whisk eggs, sour cream, vinegar, and melted butter until smooth.

Stir the wet mixture into the dry until a shaggy dough forms. Let sit 2 minutes to thicken, then transfer to the pan and smooth the top.

Bake 40–45 minutes, until deep golden and a toothpick comes out clean. Tent with foil if browning too fast.

Cool in the pan 10 minutes, then lift out and cool on a wire rack at least 30 minutes. Slice with a serrated knife and serve.