Mix the dry ingredients. In a large bowl, whisk the flours, tapioca starch, psyllium husk powder, yeast, sugar, and salt until evenly combined.

Add the wet ingredients. Add the water, egg whites, and apple cider vinegar and mix well. The psyllium will pull everything together into a soft, sticky dough.

Cold ferment overnight. Move the dough to a covered container with room to rise. I use a 1.5-quart Pyrex with a lid, or a bowl with a plate on top. Refrigerate overnight. The dough will rise to fill the container.

Shape. About 1½ hours before baking, line a quarter sheet pan with a parchment sling, with the long ends of the parchment hanging over the sides of the loaf. Turn the dough out onto the center of the parchment and gently shape it into a long slipper. To keep it tidy, you can tuck the edges of the parchment under the dough. Set a loaf pan along one long side of the loaf to keep it from spreading.

Final rise. Cover loosely and leave in a warm place until puffy and noticeably larger, about 1 to 1½ hours.

Preheat. While the dough rises, put a baking stone in the oven and preheat to 450°F for at least 45 minutes. A fully heated stone gives the most oven spring.

Prepare for steam. Uncover the loaf. If you like, dust it with rice flour or potato starch for the classic ciabatta look. Pour a little water into the loaf pan, and have a cup of water or a handful of ice cubes ready.

Bake. Put the sheet pan on the stone. Toss the water or ice onto the oven floor and close the door quickly. (If you have a gas oven with a pilot light, take care not to put it out.) Bake at 450°F for about 10 minutes, then turn the oven down to 400°F. Keep baking for another 30–40 minutes, until the crust is deep golden brown and the internal temperature reaches 210°F. Tip: If the crust browns before the inside reaches 210°F, tent the loaf loosely with foil for the rest of the bake. Tip: Ice on the oven floor can damage enamel or glass oven bottoms. If that’s a concern, put a metal pan on the bottom rack while the oven preheats and pour the water or ice into that instead.

Cool. Let the bread cool completely on a rack, at least 1 hour, before slicing. Gluten-free bread keeps setting as it cools, and slicing it warm makes the crumb gummy.