Measure the flour into a bowl and make a well in it. Crumble in the fresh yeast/sprinkle in the powdered yeast. Pour 100 g of your water over it and wait for 15 minutes or so to let it dissolve.
Sprinkle on the salt, pour on the rest of the water and get all the ingredients together in the bowl. Turn the dough out onto your work surface and knead for five minutes.
Squash in the potato and the rosemary. The dough will get very sticky and slack. Don't give up, just use a scraper to continue to gather it all up and get it off your hands and keep going.
Scrape it all up and pop it back in the bowl. Let it rest for 1-2 hours.
Pull the dough out and divide it into four blobs. Flour your hands (try to avoid flouring the work surface) and shape each blob into a tight round ball.
If you have made balls, place each ball on a baking tray that you have lined with non stick baking parchment. Cover with a tea towel.
Let rest for 1 hour.
Preheat oven to 200 degrees C (400 degrees F). Put the tray in the oven and bake for 45 minutes.
Remove from the oven and let cool completely before cutting.