Crumble the yeast in a cup, sprinkle with sugar and stir with some of the water until smooth.
Take the yogurt (or any substitute) out of the refrigerator.
Mix the flour with the salt in a large bowl. Press a hollow in the middle. Pour in the dissolved yeast and dust with a little flour from the edge. Let rest for 15 minutes.
Add the rest of the water, sourdough and yoghurt and knead everything into a smooth dough until it separates from the edge of the bowl. Cover and let rise in a warm place for 30 minutes. The dough should have enlarged significantly. (I put the sealed bowl in the microwave at 200 watts for 4 minutes and then under the covers)
Knead the dough. Shape into a ball on a floured work surface. Place the ball of dough on a baking sheet lined with baking paper or in a floured bread proofing basket. Cover and let rise again in a warm place for 30 minutes.
Preheat the oven to 220 ° C fan oven.
If necessary, turn the dough out of the bread basket onto the baking tray. Brush with water. Cut several times 1 cm deep with a sharp knife at a distance.
Slide the tray onto the lowest rail of the oven and bake for 10 minutes. Turn the temperature down to 180 ° C and bake for another 30-35 minutes.
The bread is ready when it sounds hollow on the underside of the bread when you tap it.