Attach a dough hook to an electric mixer. Put the ingredients in the bowl in the following order: dark rye flour, salt, bakers’ flour, starter and 230 ml (8 fl oz) water. Mix on low speed for 3 minutes, then medium speed for a further 2 minutes. Place the dough in a clean metal bowl and wrap with plastic wrap. Leave to prove at approximately 26°C (79°F) for 90 minutes. Gently give the dough three or four folds at 40-minute intervals to further strengthen it.
Turn out onto a clean work surface and gently fold the dough three or four times, or just enough to release any excess gas trapped in the dough. At this stage the dough will need to be covered with a damp cloth so it doesn’t dry out.
Preheat the oven to 210°C (410°F). Cut the dough into 100 g (3½ oz) pieces and roll into balls. Press out flat to about 1 cm (½ in) thick and place on semolina-dusted baking trays. Brush each roll with yoghurt, making sure you brush all the way around, from the base up. Scatter with oats.
Bake for approximately 16 minutes, steaming the oven as the bread goes in, then twice more at 3-minute intervals, by adding 125 ml (4 fl oz) water each time.