Stir sourdough, yeast and 500 ml lukewarm water. Put both types of flour and salt in a suitable bowl, add sourdough mixture and knead together with the dough hook of the mixer or possibly in a food processor.
Knead on floured surface for 7-10 min until dough is smooth and fluffy. Place in a baking bowl covered with plastic wrap in a warm place to rise for 2 hours until doubled in volume.
Divide dough into 2 portions, knead heartily and shape into oblong loaves. Place on floured baking sheet, score lengthwise, and cover with floured dishcloth. Let rise another 2 hours.
Place in hot oven at 250 °C (gas 5, convection oven 230 °C ) on the 2nd rack from the bottom, placing a deep plate of boiling hot water on the bottom of the oven. Reduce the temperature to 200 °C (gas 3, convection oven 180 °C ) and bake for 15 minutes. Now reduce again to 170 °C (gas 1-2, convection oven 150 °C ) and bake for 50 min.