In a large bowl, whisk the almond flour, coconut flour, psyllium husk powder, baking powder, and salt until no lumps remain.

Add the eggs, warm water, apple cider vinegar, and sourdough starter. Stir with a wooden spoon, then fold and press with your hands for 2 minutes until the dough turns from wet and sticky to smooth and slightly tacky.

Shape the dough into a 6-inch round on parchment, score the top with a shallow cross, and proof at warm room temperature for 2 to 3 hours until puffy and increased by one-third.

Preheat the oven to 375°F (190°C). Bake in a preheated Dutch oven (covered) for 30 minutes, then uncovered 20 to 25 minutes, or on a baking sheet with a steam pan for 50 to 55 minutes total, until deep golden and 200°F (93°C) inside.

Cool completely on a wire rack, then slice with a serrated knife and serve.