Mix 100 g flour, yeast, sugar and salt in a bowl. Add 75 ml cold water and stir until smooth. Cover and leave to rise in a warm place for about 1 1/2 hours.
Add 400 g flour and 325 ml cold water to the predough. Knead everything with the dough hooks of the hand mixer to a smooth dough. Sprinkle with oil and leave to rise overnight at room temperature, covered.
Line baking tray with baking paper. Dust the working surface thickly with flour. Put half of the dough (caution, it is very soft) on top and press carefully to form a rectangle (approx. 26 x 28 cm). Fold both long sides to the middle using a dough card and carefully place the bread on the baking tray.
Form the second half of the dough into bread in the same way and place it on the same baking tray. Bake in the preheated oven (electric cooker: 225 °C/circulating air: 200 °C/gas: level 4) for 20-25 minutes. Let the ciabatta cool down.