In a medium bowl, combine 50 grams of all-purpose flour, 2 grams of instant yeast, and 50 grams of lukewarm water. Mix until smooth, then cover with plastic wrap. Let sit at room temperature for about 3-4 hours.

Once the poolish is bubbly, add the remaining 360 grams of lukewarm water to the bowl. Sprinkle in 12 grams of salt and gradually mix in 450 grams of bread flour until a sticky dough forms. Let the dough rest for 30 minutes.

With hands slightly damp, gently stretch the dough from the side to the center of the bowl, folding it over itself. Rotate the bowl a quarter turn and repeat until a full circle is completed. Cover and let it rest for another 30 minutes.

Cover the bowl tightly with plastic wrap and let the dough rise in a warm spot until it has doubled in size, approximately 1.5-2 hours.

Once risen, gently deflate the dough and transfer it into an airtight container. Place in the fridge for 12-48 hours to develop flavors.

Turn the chilled dough out onto a lightly floured surface. Divide into two equal portions and gently shape each into a ball. Let them proof for about 2.5 hours, seam-side up, covered with a kitchen towel.

Preheat the oven to 425°F (220°C) about 20 minutes before baking. Place a pizza stone in the oven to heat up if available.

Transfer the proofed dough to a lined baking sheet or hot pizza stone. Shape into rectangles and bake for 20-25 minutes until golden brown, tapping the bottom to check for hollowness.

Remove the ciabatta loaves from the oven and let them cool on a wire rack for 20-30 minutes before cutting.